Use French (Rameau) and English (LCSH) when available in GND
As an example :
Antisemitismus http://devlinked.swissbib.ch/Page/Detail/Subject/4002333-3
In the GND, there are the equivalents :
- LCSH: Antisemitism
- RAMEAU: Antisémitisme
I would use that as a label when available and do a search with all the terms with an OR.
I know the equivalence in LCSH and RAMEAU are not always present, but that would be a first step towards multilingual subject search.