After installing Google Translate, by default only English language pack is available for offline use.
If you want to take full advantage of Google Translate offline, you need to download other language packs that are offered for offline usage. Without downloading additional packs you won't be able to translate into other specific languages and may get an error like "Translation failed", or you may not be able to get the translated text being read back to you. The iOS version of Google Translate does not supporting offline languages.
The app, if displays "Translation failed" error message, will prompt you to download the language pack. To successfully run the app and translate text into your preferred language you will have to choose the option "Download Packages" and follow instructions.
Hope that works.