I think you are worrying way too much.

Singapore is a world class airport, even my friend's mom, who does not speak much English can transfer airlines there by herself. Garuda is in T1, AF/KL is probably in T1 or T2, Walk or take the train over, it is all written in English, simple.

Interlining is fine, if a ticket is sold that involves 2 carriers, you can be sure that there is an interline agreement. I even told SQ one time to tag my bags only to SIN and not to CGK and they complied (had 8 hour layover in SIN, family is there)

I think you have to worry more about transitting at CDG than at SIN.