It’s not exactly like either of those but there is a big mystery going on in the podcast Borrasca.  But if you’re not comfortable with SA I wouldn’t recommend it.  I’m waiting patiently for season two but the first one is still really good.  

Also The Black Tapes is awesome and a bit more in line with Archive 81.  Tanis is also really good too.  Limetown is kind of a darker Nightvale but I’ve only listened to season 1.   None are French, but they’re still good.

Also unfortunately an incomplete story that won’t ever be finished.

King falls fm is the most similar to night Vale, but not  as surreal.

Hoping someone here could help me find a french podcast that is similar to welcome to nightvale... or archive 81. I havent had much luck.

Nightvale-ish