The last time I worked on a localized project, we had a technical fake language present in non-release envs. It was generated with every build (it was just a simple .po file, so easy to handle), and it was the same as English, but had an "x" at the start and end of each text, so you knew that it was "translated" (or at least it was translatable, and not hardcoded). QA included checking this fake language - if they found any text without X's around it, it was a clear sign of a i18n bug.