diff --git a/CHANGELOG.md b/CHANGELOG.md
index d85f138b6..e13ae4176 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -11,6 +11,11 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 - Upgraded `stripe` from version `21.0.1` to `22.2.3`
 
+### Fixed
+
+- Fixed the creation of the _Stripe_ checkout session for languages not supported by _Stripe_ (`ca` and `uk`)
+- Fixed the error handling in the endpoint to create a _Stripe_ checkout session
+
 ## 3.20.0 - 2026-07-04
 
 ### Changed
diff --git a/apps/api/src/app/app.module.ts b/apps/api/src/app/app.module.ts
index 0a27faa64..04b1f3bf2 100644
--- a/apps/api/src/app/app.module.ts
+++ b/apps/api/src/app/app.module.ts
@@ -147,7 +147,9 @@ import { UserModule } from './user/user.module';
                 .split(',')[0]
                 .split('-')[0];
 
-              if (SUPPORTED_LANGUAGE_CODES.includes(code)) {
+              if (
+                (SUPPORTED_LANGUAGE_CODES as readonly string[]).includes(code)
+              ) {
                 languageCode = code;
               }
             } catch {}
diff --git a/apps/api/src/app/subscription/subscription.controller.ts b/apps/api/src/app/subscription/subscription.controller.ts
index 074a9db0e..4018e4753 100644
--- a/apps/api/src/app/subscription/subscription.controller.ts
+++ b/apps/api/src/app/subscription/subscription.controller.ts
@@ -116,11 +116,11 @@ export class SubscriptionController {
 
   @Post('stripe/checkout-session')
   @UseGuards(AuthGuard('jwt'), HasPermissionGuard)
-  public createStripeCheckoutSession(
+  public async createStripeCheckoutSession(
     @Body() { couponId, priceId }: { couponId?: string; priceId: string }
   ): Promise<CreateStripeCheckoutSessionResponse> {
     try {
-      return this.subscriptionService.createStripeCheckoutSession({
+      return await this.subscriptionService.createStripeCheckoutSession({
         couponId,
         priceId,
         user: this.request.user
diff --git a/apps/api/src/app/subscription/subscription.service.ts b/apps/api/src/app/subscription/subscription.service.ts
index ab19d669a..07c2e7dbb 100644
--- a/apps/api/src/app/subscription/subscription.service.ts
+++ b/apps/api/src/app/subscription/subscription.service.ts
@@ -3,7 +3,8 @@ import { PrismaService } from '@ghostfolio/api/services/prisma/prisma.service';
 import { PropertyService } from '@ghostfolio/api/services/property/property.service';
 import {
   DEFAULT_LANGUAGE_CODE,
-  PROPERTY_STRIPE_CONFIG
+  PROPERTY_STRIPE_CONFIG,
+  SUPPORTED_LANGUAGE_CODES
 } from '@ghostfolio/common/config';
 import { SubscriptionType } from '@ghostfolio/common/enums';
 import { parseDate } from '@ghostfolio/common/helper';
@@ -75,10 +76,7 @@ export class SubscriptionService {
             quantity: 1
           }
         ],
-        locale:
-          (user.settings?.settings
-            ?.language as Stripe.Checkout.SessionCreateParams.Locale) ??
-          DEFAULT_LANGUAGE_CODE,
+        locale: this.getStripeLocale(user.settings?.settings?.language),
         metadata: subscriptionOffer
           ? { subscriptionOffer: JSON.stringify(subscriptionOffer) }
           : {},
@@ -246,4 +244,28 @@ export class SubscriptionService {
       isRenewal: key.startsWith('renewal')
     };
   }
+
+  private getStripeLocale(
+    languageCode: string
+  ): Stripe.Checkout.SessionCreateParams.Locale {
+    const unsupportedLanguageCodes: Record<
+      Exclude<
+        (typeof SUPPORTED_LANGUAGE_CODES)[number],
+        Stripe.Checkout.SessionCreateParams.Locale
+      >,
+      true
+    > = {
+      ca: true,
+      uk: true
+    };
+
+    if (
+      (SUPPORTED_LANGUAGE_CODES as readonly string[]).includes(languageCode) &&
+      !(languageCode in unsupportedLanguageCodes)
+    ) {
+      return languageCode as Stripe.Checkout.SessionCreateParams.Locale;
+    }
+
+    return DEFAULT_LANGUAGE_CODE;
+  }
 }
diff --git a/apps/api/src/middlewares/html-template.middleware.ts b/apps/api/src/middlewares/html-template.middleware.ts
index c256ada56..325f64eaa 100644
--- a/apps/api/src/middlewares/html-template.middleware.ts
+++ b/apps/api/src/middlewares/html-template.middleware.ts
@@ -117,7 +117,9 @@ export class HtmlTemplateMiddleware implements NestMiddleware {
     const path = request.originalUrl.replace(/\/$/, '');
     let languageCode = path.substr(1, 2);
 
-    if (!SUPPORTED_LANGUAGE_CODES.includes(languageCode)) {
+    if (
+      !(SUPPORTED_LANGUAGE_CODES as readonly string[]).includes(languageCode)
+    ) {
       languageCode = DEFAULT_LANGUAGE_CODE;
     }
 
diff --git a/libs/common/src/lib/config.ts b/libs/common/src/lib/config.ts
index 4f3e9dd9d..c60170bbb 100644
--- a/libs/common/src/lib/config.ts
+++ b/libs/common/src/lib/config.ts
@@ -323,7 +323,7 @@ export const SUPPORTED_LANGUAGE_CODES = [
   'tr',
   'uk',
   'zh'
-];
+] as const;
 
 export const TAG_ID_EMERGENCY_FUND = '4452656d-9fa4-4bd0-ba38-70492e31d180';
 export const TAG_ID_EXCLUDE_FROM_ANALYSIS =