Why do I need to explicitly specify the language when using nestjs-i18n in NestJS services?

Question

I'm using the nestjs-i18n package (nestjs-i18n) in my NestJS project, and I've noticed that when using i18n within a service like this:

import { Injectable } from '@nestjs/common';
import { I18nContext, I18nService } from 'nestjs-i18n';

@Injectable()
export class AppService {
  constructor(private readonly i18nService: I18nService) {}

  // I send 'x-lang=en-US' in the header
  getHello() {
    const message1 = this.i18nService.t('messages.hello'); // Returns fallback language (vi-VN) 
    const message2 = this.i18nService.t('messages.hello', {
      lang: I18nContext.current().lang, // Returns 'en-US' (correct)
    }); 
    const message3 = I18nContext.current().t('messages.hello'); // Returns 'en-US' (correct)

    return { message1, message2, message3 }; 
    /*{
      "message1": "Xin Chào",
      "message2": "Hello",
      "message3": "Hello"
     }*/
  }
}

I always have to add lang: I18nContext.current().lang to get the correct language for the current request. Why doesn't it automatically detect the appropriate language for the current request if I don't pass lang:? And here's how I've configured my AppModule:

import { Module } from '@nestjs/common';
// ... other imports 

@Module({
  imports: [
    // ... other imports
    I18nModule.forRootAsync({
      useFactory: (configService: ConfigService) => ({
        fallbackLanguage: configService.getOrThrow('FALLBACK_LANGUAGE'),
        loaderOptions: {
          path: join(__dirname, '/i18n/'),
          watch: true,
        },
      }),
      resolvers: [
        new QueryResolver(['lang', 'l']),
        new HeaderResolver(['x-lang', 'lang']),
        new CookieResolver(),
        AcceptLanguageResolver,
      ],
      inject: [ConfigService],
    }),
  ],
  // ... 
})
export class AppModule {}

I'd appreciate any insights into why this behavior occurs and how I can make the language detection more seamless. Thank you!