Basically we’re using gettext. You can find the .mo files in the install directory:
…\SteamApps\common\Train Fever\res\strings\de\LC_MESSAGES
There are some tutorials online to convert .mo files into .po files.
With a .po editor you’re able to change those translations. I’m using http://poedit.net/
Currently you can only edit an existing language (I’m not sure if english works) and preview it when selecting this language in steam.
When you’ve got a finished translations you can contact us and we can have a look at it, to get it into the game as an official translation.