Can I change the region of a save?

[DuddiSuppe]

Can I somehow convert my german ruby save into a japanese ruby save?

[SkyLink98]

You should be able to restore your save file to a japanese game normally. 

Rename the sav file correctly if using an emulator or use the GBA Backup Tool to restore it to a japanese game.

[theSLAYER]

For the case of Gen 3 and above (except Gen 4 KOR games in particular), you could just restore your save into a different languaged game of the same version.

Do note there might be issues: in particular with the OT of your save, in Gen 3, when you do cross language transfers.
The font may screw up, and retyping it may not resolve the issue.

Make sure you keep safe backups before attempting such a thing, and remember, it is all at your own risk.