diff --git a/src/app/features/project-editor/sections/general-section.component.html b/src/app/features/project-editor/sections/general-section.component.html
index c3c26ef..c88c34d 100644
--- a/src/app/features/project-editor/sections/general-section.component.html
+++ b/src/app/features/project-editor/sections/general-section.component.html
@@ -12,7 +12,7 @@
       <small class="field-desc">{{ 'builder.descriptionFieldDesc' | translate }}</small>
       <textarea rows="3" [ngModel]="bootstrap.seo.default.description" (ngModelChange)="updateDescription($event)"></textarea>
     </label>
-    <app-form-field [label]="'builder.defaultLanguage' | translate" [hint]="'builder.defaultLanguageDesc' | translate">
+    <app-form-field [label]="'builder.defaultLanguage' | translate" [hint]="'builder.defaultLanguageDesc' | translate" [error]="fieldError('localization.defaultLocale')">
       <app-input [ngModel]="bootstrap.localization.defaultLocale" (ngModelChange)="updateDefaultLanguage($event)" />
     </app-form-field>
     <app-form-field [label]="'builder.supportedLanguages' | translate" [hint]="'builder.supportedLanguagesDesc' | translate">
diff --git a/src/app/features/project-editor/sections/general-section.component.ts b/src/app/features/project-editor/sections/general-section.component.ts
index c8180a6..c303613 100644
--- a/src/app/features/project-editor/sections/general-section.component.ts
+++ b/src/app/features/project-editor/sections/general-section.component.ts
@@ -61,12 +61,25 @@ export class ProjectEditorGeneralSectionComponent {
     }));
   }
 
+  /**
+   * Routes through facade.addLocale()/removeLocale() (LocaleSyncService)
+   * instead of overwriting supportedLocales directly - a direct overwrite
+   * skipped seeding/cleaning up per-locale nav and static-page translation
+   * entries, leaving them out of sync with what this field claims is
+   * supported (and Languages' add/remove buttons already do it correctly).
+   */
   updateSupportedLanguages(value: string): void {
-    const locales = value.split(',').map(item => item.trim()).filter(Boolean);
-    this.facade.updateBootstrap(current => ({
-      ...current,
-      tenant: { ...current.tenant, supportedLocales: locales },
-      localization: { ...current.localization, supportedLocales: locales }
-    }));
+    const next = new Set(value.split(',').map(item => item.trim().toLowerCase()).filter(Boolean));
+    const current = this.bootstrap()?.localization.supportedLocales ?? [];
+    for (const locale of current) {
+      if (!next.has(locale)) {
+        this.facade.removeLocale(locale);
+      }
+    }
+    for (const locale of next) {
+      if (!current.includes(locale)) {
+        this.facade.addLocale(locale);
+      }
+    }
   }
 }
diff --git a/src/app/features/project-editor/services/project-validator.service.ts b/src/app/features/project-editor/services/project-validator.service.ts
index 23f290d..da4b69f 100644
--- a/src/app/features/project-editor/services/project-validator.service.ts
+++ b/src/app/features/project-editor/services/project-validator.service.ts
@@ -70,9 +70,17 @@ export class ProjectValidator {
   }
 
   private languageIssues(bootstrap: BootstrapConfig): ProjectValidationIssue[] {
-    return bootstrap.localization.supportedLocales.length > 0
-      ? []
-      : [error('no-languages', 'builder.validationNoLanguages', 'languages', 'localization.supportedLocales')];
+    if (bootstrap.localization.supportedLocales.length === 0) {
+      return [error('no-languages', 'builder.validationNoLanguages', 'languages', 'localization.supportedLocales')];
+    }
+    // General's free-text "Default Language" field (unlike Languages' set-default
+    // buttons, which only ever target an already-supported locale) can set this
+    // to any string. A default locale that isn't itself supported breaks every
+    // `label[defaultLocale]` lookup across nav/static-page content.
+    if (!bootstrap.localization.supportedLocales.includes(bootstrap.localization.defaultLocale)) {
+      return [error('default-locale-not-supported', 'builder.validationDefaultLocaleNotSupported', 'general', 'localization.defaultLocale')];
+    }
+    return [];
   }
 
   private urlIssues(bootstrap: BootstrapConfig): ProjectValidationIssue[] {
diff --git a/src/app/i18n/en.ts b/src/app/i18n/en.ts
index f615664..22e8288 100644
--- a/src/app/i18n/en.ts
+++ b/src/app/i18n/en.ts
@@ -538,6 +538,7 @@ export const en: Translations = {
     featureFlags: 'Feature Flags',
     validationMissingLogo: 'Branding is missing a logo.',
     validationNoLanguages: 'No languages are configured.',
+    validationDefaultLocaleNotSupported: 'The default language is not in the supported languages list.',
     validationInvalidUrl: 'The marketplace URL is invalid.',
     validationDuplicateSlugs: 'Two or more static pages share the same slug.',
     validationEmptyHomepage: 'The homepage has no sections.',
diff --git a/src/app/i18n/hy.ts b/src/app/i18n/hy.ts
index 10af5cd..003379f 100644
--- a/src/app/i18n/hy.ts
+++ b/src/app/i18n/hy.ts
@@ -538,6 +538,7 @@ export const hy: Translations = {
     featureFlags: 'Feature flags',
     validationMissingLogo: 'Բրենդինգում բացակայում է լոգոն։',
     validationNoLanguages: 'Կարգավորված լեզուներ չկան։',
+    validationDefaultLocaleNotSupported: 'Կանխադրված լեզուն աջակցվող լեզուների ցանկում չէ։',
     validationInvalidUrl: 'Մարքեթփլեյսի URL-ը սխալ է։',
     validationDuplicateSlugs: 'Երկու կամ ավելի ստատիկ էջեր ունեն նույն slug-ը։',
     validationEmptyHomepage: 'Գլխավոր էջում սեկցիաներ չկան։',
diff --git a/src/app/i18n/ru.ts b/src/app/i18n/ru.ts
index e57f0a9..bf26a79 100644
--- a/src/app/i18n/ru.ts
+++ b/src/app/i18n/ru.ts
@@ -538,6 +538,7 @@ export const ru: Translations = {
     featureFlags: 'Фичи-флаги',
     validationMissingLogo: 'В брендинге отсутствует логотип.',
     validationNoLanguages: 'Не настроены языки.',
+    validationDefaultLocaleNotSupported: 'Язык по умолчанию отсутствует в списке поддерживаемых языков.',
     validationInvalidUrl: 'Некорректный URL маркетплейса.',
     validationDuplicateSlugs: 'Две или более статические страницы имеют одинаковый slug.',
     validationEmptyHomepage: 'На главной странице нет секций.',
diff --git a/src/app/i18n/translations.ts b/src/app/i18n/translations.ts
index 488dae0..ff4d576 100644
--- a/src/app/i18n/translations.ts
+++ b/src/app/i18n/translations.ts
@@ -536,6 +536,7 @@ export interface Translations {
     featureFlags: string;
     validationMissingLogo: string;
     validationNoLanguages: string;
+    validationDefaultLocaleNotSupported: string;
     validationInvalidUrl: string;
     validationDuplicateSlugs: string;
     validationEmptyHomepage: string;