feat(auth): use browser language preference instead of IP-based language (#3517)

* feat(auth): use browser language preference instead of IP-based language

- Add optional `language` parameter to SignUpDto
- Pass browser language from frontend to createUser
- Prefer browser language over IP-based country language
- Remove country-specific language defaults (CH/LI/DE/AT/IT/FR)
- Keep only currency defaults for CH/LI (CHF)

This fixes the issue where Swiss users always got German regardless
of their browser language setting. Language should be determined by
user preference (browser), not by geographic location.

* chore: cleanup

---------

Co-authored-by: David May <david.leo.may@gmail.com>

Author: TaprootFreak

src/config/config.ts

@@ -66,7 +66,8 @@ export class Configuration {
     language: 'EN',
 
     specific: {
-      CH: { language: 'DE', currency: 'CHF' },
+      // CH has no language default (multilingual country)
+      CH: { currency: 'CHF' },
       LI: { language: 'DE', currency: 'CHF' },
       DE: { language: 'DE' },
       AT: { language: 'DE' },

src/subdomains/generic/user/models/auth/auth.service.ts

@@ -173,6 +173,7 @@ export class AuthService {
       },
       dto.specialCode ?? dto.discountCode,
       dto.moderator,
+      dto.language,
     );
 
     // update ip Logs

src/subdomains/generic/user/models/auth/dto/auth-credentials.dto.ts

@@ -105,6 +105,11 @@ export class OptionalSignUpDto {
 }
 
 export class SignUpDto extends SignInDto {
+  @ApiPropertyOptional({ description: 'Preferred language (e.g. EN, DE, FR, IT)' })
+  @IsOptional()
+  @IsString()
+  language?: string;
+
   @ApiPropertyOptional()
   @IsOptional()
   @IsString()

src/subdomains/generic/user/models/user/user.service.ts

@@ -244,7 +244,12 @@ export class UserService {
     return UserDtoMapper.mapProfile(userData);
   }
 
-  async createUser(data: Partial<User>, specialCode: string, moderator?: Moderator): Promise<User> {
+  async createUser(
+    data: Partial<User>,
+    specialCode: string,
+    moderator?: Moderator,
+    preferredLanguage?: string,
+  ): Promise<User> {
     let user = this.userRepo.create({
       address: data.address,
       signature: data.signature,
@@ -267,7 +272,9 @@ export class UserService {
     user.role = data.role;
     user.primaryUser = data.primaryUser;
 
-    const language = await this.languageService.getLanguageByCountry(user.ipCountry);
+    const language =
+      (preferredLanguage && (await this.languageService.getLanguageBySymbol(preferredLanguage.toUpperCase()))) ??
+      (await this.languageService.getLanguageByCountry(user.ipCountry));
     const currency = await this.fiatService.getFiatByCountry(user.ipCountry);
 
     user.userData =