West Germanic underwent a sound change where consonants followed
by /j/ where lengthened, /CjV/ > /CCjV/. That always seemed a bit
peculiar to me. What happens phonetically there?

I haven't found the time to read a history of Italian yet, but
clearly Italian experienced a similar change, e.g. subjunctive
faciat > faccia, sapiat > sappia.

Also, the Western Romance forms look suspiciously like reflexes of
a geminate, too, pushing this at least partly into Vulgar Latin ~
Proto-Romance territory:
facit > French fait, Spanish hace
faciat > French fasse, Spanish haga
sapit > French sait, Spanish sabe
sapiat > French sache, Spanish sepa

The areal and temporal vicinity makes me wonder just how independent
that sound change is from the West Germanic one.

Are there examples of /j/ triggering gemination in more remotely
or unrelated languages?

--
Christian "naddy" Weisgerber naddy@mips.inka.de