Too late for the party, but this may interest people.
Something I'll try if my sucky tentative to keep the Light of Andorra alive (albeit through a spin-of) works:
If learning a language you already have a default with, spend xp normaly, as if starting from scratch. Add this to the default to determine your score in the new language, with a maximum being equal to the original language's score. E.g, I have High German 05, giving me Low German 03. If I put 05 xp into Low german, for a score of 1, I can speak it at 3+1 = 4.
I haven't thought about what happens if you put too much points (High German 05, Low German 03), I'll solve that should I need to, but I figure it should work well enough for characters who want to improve in a lot of related languages.