Hi Stefan,
I find a way to do this by changing XML files, first you have to export your flavor using /PERSONAS/FLAVORS, this generate a .zip file with some files inside, you have to edit the following files and change the language tag:
For example in my case i want to change from English to Spanish.
changestexts.YOURFLAVORID.xml
<SPRAS>E</SPRAS> I change the language key from E to S, <SPRAS>S</SPRAS>.
flavor.YOURFLAVORID.xml
change <ORIG_LANG>E</ORIG_LANG> tag to <ORIG_LANG>S</ORIG_LANG>
flavortexts.YOURFLAVORID.xml
change <SPRAS>E</SPRAS> tag to <SPRAS>S</SPRAS>
You could use and text editor (if using windows use notepad), and use search and replace tool because it could be that there are more than one occurrence of the tags.
Once you have edited the files you have to import again using the same transaction /PERSONAS/FLAVORS. Be aware that the system will try to overwrite your flavor because the ID are the same, so think first in make a backup of your flavor.
So far this is the trick I did to change the original language.
Kind regards.
Jhon Jairo Teran.