Why not taking option 1 (which would be far cleaner),

Ok, I agree. Since Zeno Tajoli raises the issue, if he want to do the job, I pass. Otherwise, I will submit a patch. Zeno ?

provided that time is invested in order to make C4::XSLT to take the correct language path
for XSLT

Isn't it the case already :

http://tinyurl.com/248dbpe
line 156