You will face the dark side of the bootstrap package when you try to use different languages than those configured as examples. There's only a mapping between the value of sys_language_id and English, German and Danish available and if you try to add a mapping manually, RealURL immediately switches to the "manual mode" where you have to configure everything yourself once again.
Loosing the auto-configuration implies a lot of work, especially if you are using the Aimeos web shop extension or similar ones, which add a lot of RealURL rules out of the box. Thus, it's very compelling to try to overwrite only the RealURL configuration which is responsible for the language ID/code mapping.