diff --git a/.changeset/chilly-otters-beam.md b/.changeset/chilly-otters-beam.md
new file mode 100644
index 000000000..77341e6bf
--- /dev/null
+++ b/.changeset/chilly-otters-beam.md
@@ -0,0 +1,5 @@
+---
+"@gitbook/colors": patch
+---
+
+Fix return type for `colorContrast`
diff --git a/.changeset/red-countries-confess.md b/.changeset/red-countries-confess.md
new file mode 100644
index 000000000..e334b3280
--- /dev/null
+++ b/.changeset/red-countries-confess.md
@@ -0,0 +1,5 @@
+---
+"gitbook": patch
+---
+
+Use space language as source of truth for UI locale
diff --git a/packages/colors/src/transformations.ts b/packages/colors/src/transformations.ts
index 82d09e011..e6773039c 100644
--- a/packages/colors/src/transformations.ts
+++ b/packages/colors/src/transformations.ts
@@ -419,7 +419,10 @@ export function dpsContrast(a: RGBColor, b: RGBColor) {
     return contrast < 7.5 ? 0 : contrast;
 }
 
-export function colorContrast(background: string, foreground: string[] = [LIGHT_BASE, DARK_BASE]) {
+export function colorContrast(
+    background: string,
+    foreground: string[] = [LIGHT_BASE, DARK_BASE]
+): string {
     const bg = hexToRgbArray(background);
 
     const best: { color?: RGBColor; contrast: number } = {
@@ -436,5 +439,5 @@ export function colorContrast(background: string, foreground: string[] = [LIGHT_
         }
     }
 
-    return best.color ? rgbArrayToHex(best.color) : foreground[0];
+    return best.color ? rgbArrayToHex(best.color) : foreground[0] || LIGHT_BASE;
 }
diff --git a/packages/gitbook/src/app/sites/dynamic/[mode]/[siteURL]/[siteData]/(content)/layout.tsx b/packages/gitbook/src/app/sites/dynamic/[mode]/[siteURL]/[siteData]/(content)/layout.tsx
index a8fd4e551..eea0f8516 100644
--- a/packages/gitbook/src/app/sites/dynamic/[mode]/[siteURL]/[siteData]/(content)/layout.tsx
+++ b/packages/gitbook/src/app/sites/dynamic/[mode]/[siteURL]/[siteData]/(content)/layout.tsx
@@ -22,7 +22,7 @@ export default async function SiteDynamicLayout({
     const withTracking = shouldTrackEvents(await headers());
 
     return (
-        <CustomizationRootLayout forcedTheme={forcedTheme} customization={context.customization}>
+        <CustomizationRootLayout forcedTheme={forcedTheme} context={context}>
             <SiteLayout
                 context={context}
                 forcedTheme={forcedTheme}
diff --git a/packages/gitbook/src/app/sites/static/[mode]/[siteURL]/[siteData]/(content)/layout.tsx b/packages/gitbook/src/app/sites/static/[mode]/[siteURL]/[siteData]/(content)/layout.tsx
index eaf53e18b..97bec81b3 100644
--- a/packages/gitbook/src/app/sites/static/[mode]/[siteURL]/[siteData]/(content)/layout.tsx
+++ b/packages/gitbook/src/app/sites/static/[mode]/[siteURL]/[siteData]/(content)/layout.tsx
@@ -19,7 +19,7 @@ export default async function SiteStaticLayout({
     const withTracking = shouldTrackEvents();
 
     return (
-        <CustomizationRootLayout customization={context.customization}>
+        <CustomizationRootLayout context={context}>
             <SiteLayout
                 context={context}
                 withTracking={withTracking}
diff --git a/packages/gitbook/src/components/AI/server-actions/AIToolCallsSummary.tsx b/packages/gitbook/src/components/AI/server-actions/AIToolCallsSummary.tsx
index 6432ee2d0..290081f69 100644
--- a/packages/gitbook/src/components/AI/server-actions/AIToolCallsSummary.tsx
+++ b/packages/gitbook/src/components/AI/server-actions/AIToolCallsSummary.tsx
@@ -72,7 +72,7 @@ function DescriptionForPageContentToolCall(props: {
 }) {
     const { toolCall, context } = props;
 
-    const language = getSpaceLanguage(context.customization);
+    const language = getSpaceLanguage(context);
 
     return (
         <p>
@@ -102,7 +102,7 @@ function DescriptionForMCPToolCall(props: {
 }) {
     const { toolCall, context } = props;
 
-    const language = getSpaceLanguage(context.customization);
+    const language = getSpaceLanguage(context);
 
     return (
         <p>
@@ -130,7 +130,7 @@ async function DescriptionForSearchToolCall(props: {
 }) {
     const { toolCall, context } = props;
 
-    const language = getSpaceLanguage(context.customization);
+    const language = getSpaceLanguage(context);
 
     // Resolve all hrefs for search results in parallel
     const searchResultsWithHrefs = await Promise.all(
@@ -241,7 +241,7 @@ function DescriptionForGetPagesToolCall(props: {
 }) {
     const { toolCall, context } = props;
 
-    const language = getSpaceLanguage(context.customization);
+    const language = getSpaceLanguage(context);
 
     return (
         <p>
diff --git a/packages/gitbook/src/components/DocumentView/InlineLink/InlineLink.tsx b/packages/gitbook/src/components/DocumentView/InlineLink/InlineLink.tsx
index 33e730403..e0869e3e9 100644
--- a/packages/gitbook/src/components/DocumentView/InlineLink/InlineLink.tsx
+++ b/packages/gitbook/src/components/DocumentView/InlineLink/InlineLink.tsx
@@ -20,10 +20,7 @@ export async function InlineLink(props: InlineProps<DocumentInlineLink>) {
         : null;
     const { contentContext } = context;
 
-    const language =
-        contentContext && 'customization' in contentContext
-            ? getSpaceLanguage(contentContext.customization)
-            : languages.en;
+    const language = contentContext ? getSpaceLanguage(contentContext) : languages.en;
 
     if (!contentContext || !resolved) {
         return (
diff --git a/packages/gitbook/src/components/DocumentView/OpenAPI/context.tsx b/packages/gitbook/src/components/DocumentView/OpenAPI/context.tsx
index 2dc088f24..70be2eb45 100644
--- a/packages/gitbook/src/components/DocumentView/OpenAPI/context.tsx
+++ b/packages/gitbook/src/components/DocumentView/OpenAPI/context.tsx
@@ -11,7 +11,7 @@ import { Heading } from '../Heading';
 
 import './scalar.css';
 import './style.css';
-import { DEFAULT_LOCALE, getCustomizationLocale } from '@/intl/server';
+import { DEFAULT_LOCALE, getSpaceLocale } from '@/intl/server';
 import type { GitBookAnyContext } from '@/lib/context';
 import type {
     AnyOpenAPIOperationsBlock,
@@ -30,10 +30,7 @@ export function getOpenAPIContext(args: {
     const { props, specUrl, context } = args;
     const { block } = props;
 
-    const customization = context && 'customization' in context ? context.customization : null;
-    const customizationLocale = customization
-        ? getCustomizationLocale(customization)
-        : DEFAULT_LOCALE;
+    const customizationLocale = context ? getSpaceLocale(context) : DEFAULT_LOCALE;
     const locale = checkIsValidLocale(customizationLocale) ? customizationLocale : DEFAULT_LOCALE;
 
     return {
diff --git a/packages/gitbook/src/components/Embeddable/EmbeddableRootLayout.tsx b/packages/gitbook/src/components/Embeddable/EmbeddableRootLayout.tsx
index 3e2ecd1ff..fec1269d4 100644
--- a/packages/gitbook/src/components/Embeddable/EmbeddableRootLayout.tsx
+++ b/packages/gitbook/src/components/Embeddable/EmbeddableRootLayout.tsx
@@ -21,7 +21,7 @@ export async function EmbeddableRootLayout({
     children,
 }: React.PropsWithChildren<EmbeddableRootLayoutProps>) {
     return (
-        <CustomizationRootLayout customization={context.customization}>
+        <CustomizationRootLayout context={context}>
             <SiteLayoutClientContexts
                 forcedTheme={context.customization.themes.default}
                 externalLinksTarget={context.customization.externalLinks.target}
diff --git a/packages/gitbook/src/components/Header/Header.tsx b/packages/gitbook/src/components/Header/Header.tsx
index 79e0ea926..f9a32ee88 100644
--- a/packages/gitbook/src/components/Header/Header.tsx
+++ b/packages/gitbook/src/components/Header/Header.tsx
@@ -138,7 +138,7 @@ export function Header(props: {
                                         );
                                     })}
                                     <HeaderLinkMore
-                                        label={t(getSpaceLanguage(customization), 'more')}
+                                        label={t(getSpaceLanguage(context), 'more')}
                                         links={customization.header.links}
                                         context={context}
                                     />
diff --git a/packages/gitbook/src/components/PDF/PDFPage.tsx b/packages/gitbook/src/components/PDF/PDFPage.tsx
index d29790f25..3918283c1 100644
--- a/packages/gitbook/src/components/PDF/PDFPage.tsx
+++ b/packages/gitbook/src/components/PDF/PDFPage.tsx
@@ -57,7 +57,7 @@ export async function PDFPage(props: {
 
     const customization =
         'customization' in baseContext ? baseContext.customization : defaultCustomization();
-    const language = getSpaceLanguage(customization);
+    const language = getSpaceLanguage(baseContext);
 
     // Compute the pages to render
     const { pages, total } = selectPages(baseContext.revision.pages, pdfParams);
@@ -149,8 +149,7 @@ export async function PDFPage(props: {
                 trademark={
                     customization.trademark.enabled ? (
                         <TrademarkLink
-                            space={context.space}
-                            customization={customization}
+                            context={context}
                             placement={SiteInsightsTrademarkPlacement.Pdf}
                         />
                     ) : null
diff --git a/packages/gitbook/src/components/PDF/PDFRootLayout.tsx b/packages/gitbook/src/components/PDF/PDFRootLayout.tsx
index 8b290e2d1..498adbeb8 100644
--- a/packages/gitbook/src/components/PDF/PDFRootLayout.tsx
+++ b/packages/gitbook/src/components/PDF/PDFRootLayout.tsx
@@ -1,6 +1,5 @@
 import { CustomizationRootLayout } from '@/components/RootLayout';
 import type { GitBookSiteContext, GitBookSpaceContext } from '@/lib/context';
-import { defaultCustomization } from '@/lib/utils';
 import { CustomizationThemeMode } from '@gitbook/api';
 
 /**
@@ -13,12 +12,7 @@ export async function PDFRootLayout(props: {
     const { context, children } = props;
 
     return (
-        <CustomizationRootLayout
-            customization={
-                'customization' in context ? context.customization : defaultCustomization()
-            }
-            forcedTheme={CustomizationThemeMode.Light}
-        >
+        <CustomizationRootLayout context={context} forcedTheme={CustomizationThemeMode.Light}>
             {children}
         </CustomizationRootLayout>
     );
diff --git a/packages/gitbook/src/components/PageAside/PageAside.tsx b/packages/gitbook/src/components/PageAside/PageAside.tsx
index 3ab7b2220..bc0570c41 100644
--- a/packages/gitbook/src/components/PageAside/PageAside.tsx
+++ b/packages/gitbook/src/components/PageAside/PageAside.tsx
@@ -128,7 +128,7 @@ export function PageAside(props: {
 
 function PageAsideHeader(props: { context: GitBookSiteContext }) {
     const { context } = props;
-    const language = getSpaceLanguage(context.customization);
+    const language = getSpaceLanguage(context);
 
     return (
         <div
diff --git a/packages/gitbook/src/components/PageBody/PageBody.tsx b/packages/gitbook/src/components/PageBody/PageBody.tsx
index 3a9f420c1..10c4e7a1c 100644
--- a/packages/gitbook/src/components/PageBody/PageBody.tsx
+++ b/packages/gitbook/src/components/PageBody/PageBody.tsx
@@ -42,7 +42,7 @@ export function PageBody(props: {
         : false;
     const pageWidthWide = page.layout.width === 'wide';
     const siteWidthWide = pageWidthWide || contentFullWidth;
-    const language = getSpaceLanguage(customization);
+    const language = getSpaceLanguage(context);
     const updatedAt = page.updatedAt ?? page.createdAt;
 
     return (
diff --git a/packages/gitbook/src/components/PageBody/PageFooterNavigation.tsx b/packages/gitbook/src/components/PageBody/PageFooterNavigation.tsx
index 7df3c295f..61e908af7 100644
--- a/packages/gitbook/src/components/PageBody/PageFooterNavigation.tsx
+++ b/packages/gitbook/src/components/PageBody/PageFooterNavigation.tsx
@@ -17,9 +17,9 @@ export async function PageFooterNavigation(props: {
     page: RevisionPageDocument;
 }) {
     const { context, page } = props;
-    const { customization, revision, linker } = context;
+    const { revision, linker } = context;
     const { previous, next } = resolvePrevNextPages(revision.pages, page);
-    const language = getSpaceLanguage(customization);
+    const language = getSpaceLanguage(context);
     const previousHref = previous
         ? linker.toPathForPage({ pages: revision.pages, page: previous })
         : '';
diff --git a/packages/gitbook/src/components/RootLayout/CustomizationRootLayout.tsx b/packages/gitbook/src/components/RootLayout/CustomizationRootLayout.tsx
index 9f86cdf4f..3fd1290bd 100644
--- a/packages/gitbook/src/components/RootLayout/CustomizationRootLayout.tsx
+++ b/packages/gitbook/src/components/RootLayout/CustomizationRootLayout.tsx
@@ -31,7 +31,9 @@ import { RootLayoutClientContexts } from './RootLayoutClientContexts';
 
 import '@gitbook/icons/style.css';
 import './globals.css';
+import type { GitBookAnyContext } from '@/lib/context';
 import { GITBOOK_FONTS_URL, GITBOOK_ICONS_TOKEN, GITBOOK_ICONS_URL } from '@/lib/env';
+import { defaultCustomization } from '@/lib/utils';
 import { AnnouncementDismissedScript } from '../Announcement';
 
 function preloadFont(fontData: FontData) {
@@ -56,12 +58,14 @@ function preloadFont(fontData: FontData) {
  */
 export async function CustomizationRootLayout(props: {
     forcedTheme?: CustomizationThemeMode | null;
-    customization: SiteCustomizationSettings;
+    context: GitBookAnyContext;
     children: React.ReactNode;
 }) {
-    const { customization, forcedTheme, children } = props;
+    const { context, forcedTheme, children } = props;
+    const customization =
+        'customization' in context ? context.customization : defaultCustomization();
 
-    const language = getSpaceLanguage(customization);
+    const language = getSpaceLanguage(context);
     const tintColor = getTintColor(customization);
     const mixColor = getTintMixColor(customization.styling.primaryColor, tintColor);
     const sidebarStyles = getSidebarStyles(customization);
@@ -141,7 +145,6 @@ export async function CustomizationRootLayout(props: {
                             )
                         };
                         --header-link: ${hexToRgb(
-                            // @ts-expect-error
                             customization.header.linkColor?.light ??
                                 colorContrast(
                                     tintColor?.light ?? customization.styling.primaryColor.light
@@ -161,7 +164,6 @@ export async function CustomizationRootLayout(props: {
 
                         --header-background: ${hexToRgb(customization.header.backgroundColor?.dark ?? tintColor?.dark ?? customization.styling.primaryColor.dark)};
                         --header-link: ${hexToRgb(
-                            // @ts-expect-error
                             customization.header.linkColor?.dark ??
                                 colorContrast(
                                     tintColor?.dark ?? customization.styling.primaryColor.dark
@@ -337,7 +339,6 @@ function generateColorVariable(
     return Object.entries(shades)
         .map(([key, value]) => {
             const rgbValue = hexToRgb(value); // Check the original hex value
-            // @ts-expect-error
             const contrastValue = withContrast ? hexToRgb(colorContrast(value)) : undefined; // Add contrast if needed
             return `--${name}-${key}: ${rgbValue}; ${
                 contrastValue ? `--contrast-${name}-${key}: ${contrastValue};` : ''
diff --git a/packages/gitbook/src/components/TableOfContents/TableOfContents.tsx b/packages/gitbook/src/components/TableOfContents/TableOfContents.tsx
index 426c6e9cb..9afcc33d1 100644
--- a/packages/gitbook/src/components/TableOfContents/TableOfContents.tsx
+++ b/packages/gitbook/src/components/TableOfContents/TableOfContents.tsx
@@ -16,7 +16,7 @@ export async function TableOfContents(props: {
     innerHeader?: React.ReactNode; // Displayed outside the scrollable TOC, directly above the page list
 }) {
     const { innerHeader, context, header } = props;
-    const { space, customization, revision } = context;
+    const { customization, revision } = context;
 
     const pages = await encodeClientTableOfContents(context, revision.pages, revision.pages);
 
@@ -115,8 +115,7 @@ export async function TableOfContents(props: {
                         />
                         {customization.trademark.enabled ? (
                             <Trademark
-                                space={space}
-                                customization={customization}
+                                context={context}
                                 placement={SiteInsightsTrademarkPlacement.Sidebar}
                             />
                         ) : null}
diff --git a/packages/gitbook/src/components/TableOfContents/Trademark.tsx b/packages/gitbook/src/components/TableOfContents/Trademark.tsx
index 487568555..bd077d84c 100644
--- a/packages/gitbook/src/components/TableOfContents/Trademark.tsx
+++ b/packages/gitbook/src/components/TableOfContents/Trademark.tsx
@@ -1,21 +1,17 @@
-import type {
-    SiteCustomizationSettings,
-    SiteInsightsTrademarkPlacement,
-    Space,
-} from '@gitbook/api';
+import type { SiteInsightsTrademarkPlacement } from '@gitbook/api';
 import { Icon } from '@gitbook/icons';
 
 import { getSpaceLanguage, t } from '@/intl/server';
 import { tcls } from '@/lib/tailwind';
 
+import type { GitBookSpaceContext } from '@/lib/context';
 import { Link } from '../primitives';
 
 /**
  * Trademark link to the GitBook.
  */
 export function Trademark(props: {
-    space: Space;
-    customization: SiteCustomizationSettings;
+    context: GitBookSpaceContext;
     placement: SiteInsightsTrademarkPlacement;
 }) {
     return (
@@ -71,12 +67,12 @@ export function Trademark(props: {
  * Trademark link to the GitBook.
  */
 export function TrademarkLink(props: {
-    space: Space;
-    customization: SiteCustomizationSettings;
+    context: GitBookSpaceContext;
     placement: SiteInsightsTrademarkPlacement;
 }) {
-    const { space, customization, placement } = props;
-    const language = getSpaceLanguage(customization);
+    const { context, placement } = props;
+    const { space } = context;
+    const language = getSpaceLanguage(context);
 
     const url = new URL('https://www.gitbook.com');
     url.searchParams.set('utm_source', 'content');
diff --git a/packages/gitbook/src/intl/server.ts b/packages/gitbook/src/intl/server.ts
index b13076f41..a6ed78a6c 100644
--- a/packages/gitbook/src/intl/server.ts
+++ b/packages/gitbook/src/intl/server.ts
@@ -1,25 +1,44 @@
-import type { SiteCustomizationSettings } from '@gitbook/api';
-
+import type { GitBookAnyContext } from '@/lib/context';
 import { type TranslationLanguage, languages } from './translations';
 
 export * from './translate';
 
-export const DEFAULT_LOCALE = 'en';
+type TranslationLocale = keyof typeof languages;
+
+export const DEFAULT_LOCALE = 'en' satisfies TranslationLocale;
 
 /**
- * Get the locale of the customization.
+ * Get the locale to use for a space.
  */
-export function getCustomizationLocale(customization: SiteCustomizationSettings): string {
-    return customization.internationalization.locale;
+export function getSpaceLocale(context: GitBookAnyContext): TranslationLocale {
+    const { space } = context;
+    const customization = 'site' in context ? context.customization : null;
+
+    // If the language is configured in the space, use it in priority
+    if (space.language) {
+        if (checkIsValidLocale(space.language)) {
+            return space.language;
+        }
+        return DEFAULT_LOCALE;
+    }
+
+    // Otherwise fallback to the deprecated customization settings
+    if (customization) {
+        return checkIsValidLocale(customization.internationalization.locale)
+            ? customization.internationalization.locale
+            : DEFAULT_LOCALE;
+    }
+
+    return DEFAULT_LOCALE;
 }
 
 /**
  * Create the translation context for a space to use in the server components.
  */
-export function getSpaceLanguage(customization: SiteCustomizationSettings): TranslationLanguage {
+export function getSpaceLanguage(context: GitBookAnyContext): TranslationLanguage {
     const fallback = languages[DEFAULT_LOCALE];
 
-    const locale = getCustomizationLocale(customization);
+    const locale = getSpaceLocale(context);
 
     let language = fallback;
     // @ts-ignore
@@ -33,3 +52,7 @@ export function getSpaceLanguage(customization: SiteCustomizationSettings): Tran
         ...language,
     };
 }
+
+function checkIsValidLocale(locale: string): locale is TranslationLocale {
+    return locale in languages;
+}
diff --git a/packages/gitbook/src/intl/translations/index.ts b/packages/gitbook/src/intl/translations/index.ts
index 06fc41268..be9500316 100644
--- a/packages/gitbook/src/intl/translations/index.ts
+++ b/packages/gitbook/src/intl/translations/index.ts
@@ -1,5 +1,3 @@
-import type { CustomizationLocale } from '@gitbook/api';
-
 import { de } from './de';
 import { en } from './en';
 import { es } from './es';
@@ -14,11 +12,7 @@ import { zh } from './zh';
 
 export * from './types';
 
-export const languages: {
-    [key: string]: TranslationLanguage;
-} & {
-    [locale in CustomizationLocale]: TranslationLanguage;
-} = {
+export const languages = {
     de,
     en,
     fr,
@@ -29,4 +23,6 @@ export const languages: {
     no,
     'pt-br': pt_br,
     ru,
+} satisfies {
+    [key: string]: TranslationLanguage;
 };