Are you talking about the onboard computer? I have no idea what age BMW you have, but looking in the owner's manual is always a good idea--it should be explained there.
For my '86 5-Series, you did have to select a language after the battery was reconnected. I believe the OBC said "UHR" (German for time), and from there you may have had to use a paper clip to cycle through the languages--I can't quite remember. You also choose metric vs. standard units.
I hope this is what you're asking about
