async activateSpellchecker (lang) {
    try {
      this.enabled = true
      this.isProviderAvailable = true
      if (isOsx) {
        // No language string needed on macOS.
        return await ipcRenderer.invoke('mt::spellchecker-set-enabled', true)
      }
      return await this.switchLanguage(lang || this.currentSpellcheckerLanguage)
    } catch (error) {
      this.deactivateSpellchecker()
      throw error
    }
  }
Enable the spell checker and sets `lang` or tries to find a fallback.

@param {string} lang The language to set.
@returns {Promise<boolean>}