i18n() is very sensitive on white-spaces, hidden extra chars, etc. Make sure that your translation.ts file is "clean". Remember to clear all the cache.
@Lukasz: I cannot figure out any redundant whitespaces. The posting above including the content of the translation.ts file has been created using copy&paste, and there is no wrong character inside, I think.
@Claudia: This was my fault while typing this posting, the real file names are fine.