stf-DeviceFarmer/res/app/settings/language/language-service.js

var supportedLanguages = require('./../../../common/lang/langs.json')

module.exports = function LanguageServiceFactory(
  SettingsService
, gettextCatalog
) {
  var LanguageService = {}

  function detectLanguage() {
    return (navigator.language || navigator.userLanguage || 'en-US')
      .substring(0, 2)
  }

  function isSupported(lang) {
    return !!supportedLanguages[lang]
  }

  function onlySupported(lang, defaultValue) {
    return isSupported(lang) ? lang : defaultValue
  }

  LanguageService.settingKey = 'selectedLanguage'
  LanguageService.supportedLanguages = supportedLanguages
  LanguageService.defaultLanguage = 'en'
  LanguageService.detectedLanguage =
    onlySupported(detectLanguage(), LanguageService.defaultLanguage)

  SettingsService.sync(
    LanguageService
  , {
      target: LanguageService.settingKey
    , source: LanguageService.settingKey
    , defaultValue: LanguageService.detectedLanguage
    }
  , updateLanguage
  )

  function updateLanguage() {
    gettextCatalog.setCurrentLanguage(LanguageService.selectedLanguage)
  }

  LanguageService.updateLanguage = updateLanguage

  return LanguageService
}