feature: localization's first implmentation

backend/src/modules/auth/dto/update-language.dto.ts

@@ -0,0 +1,14 @@
+import { ApiProperty } from '@nestjs/swagger';
+import { IsIn, IsString } from 'class-validator';
+
+export const SUPPORTED_USER_LANGUAGES = ['en', 'fa', 'nl'] as const;
+export type SupportedUserLanguage = (typeof SUPPORTED_USER_LANGUAGES)[number];
+
+export class UpdateLanguageDto {
+  @ApiProperty({ enum: SUPPORTED_USER_LANGUAGES, example: 'en' })
+  @IsString()
+  @IsIn(SUPPORTED_USER_LANGUAGES, {
+    message: 'Language must be one of: en, fa, nl',
+  })
+  language: SupportedUserLanguage;
+}