Coordinate system doesn't contain coordinates in the range -180 to 180 respective -90 to 90?

Hello!

I'm creating an android version of a PC program (I've contacted the complany who owns the PC program, so it's legal). The program is in the core a GPS, but is used to navigate pre-defined routes for heavyweight transports, because they are not allowed to pass some bridges. However, that isn't very important for my problem. The program reads the coordinates from text files, so my app needs to do that too, but those coordinates are not in the range -180 to 180 respective -90 to 90. So I can't really understand them. For example, one coordinate has these values:

X: 6603939,57000491

Y: 575841,753705909

I need to convert these coordinates to "normal" coordinates (latitude and longitude, in the range -180 to 180 respective -90 to 90) in order to be able to use them for what I want. So any explaination or solution is greatly appreciated!

Thanks!