如果使用@ngx-translate，则app.module.ts应如下所示

import ...
import { TranslateModule, TranslateLoader } from '@ngx-translate/core';
import { TranslateHttpLoader } from '@ngx-translate/http-loader';
import { HttpClient, HttpClientModule } from '@angular/common/http';

export function HttpLoaderFactory(http: HttpClient): TranslateHttpLoader {
  return new TranslateHttpLoader(http, './assets/i18n/', '.json');
}

@NgModule({
    declarations: [
        AppComponent
    ],
    imports: [
        ...
        TranslateModule.forRoot({
            loader: {
                provide: TranslateLoader,
                useFactory: HttpLoaderFactory,
                deps: [HttpClient]
            }
        })
    ],
    providers: [
        ...
    ],
    bootstrap: [AppComponent]
})
export class AppModule { }

任何使用转换的页面组件的模块都应该如下所示

import ...
import { TranslateModule } from '@ngx-translate/core';

@NgModule({
  imports: [
    ...,
    TranslateModule
  ],
  declarations: [SomePage]
})
export class SomePageModule {}

离线翻译应该没问题。
您可以参考我的Ionic app project，它支持多种语言。

编辑1：

如果您必须在选取翻译加载程序之前检查条件（例如网络），自定义TranslateHttpLoader可能会帮助您。
该类应如下所示

import { HttpClient } from "@angular/common/http";
import { TranslateLoader } from "@ngx-translate/core";
import { Observable } from 'rxjs';

export class TranslateHttpLoader implements TranslateLoader {
  constructor(private http: HttpClient, public prefix: string, public suffix: string) 
  {
    // Your logic
  }

  public getTranslation(lang: string): Observable<Object> {
    if (Your logic)
      return this.http.get(`${local_path}`);
    else
      return this.http.get(`${online_path}`);
  }
}

然后在app.module.ts中导入

TranslateModule.forRoot({
  loader: {
    provide: TranslateLoader,
    useFactory: HttpLoaderFactory, // <-- Here
    deps: [HttpClient]
  }
})