Why is the slavic languages or people reffered to as Cannanim in Jewish sources? Who is the first to do this?
See for instance Minchas Yehuda in Mechokekei Yehuda explaining Rashi on Devarim 3:9 Here. Minchas Yehuda says this was a common term for Slavic by many accounts, but he points in particular to HaYehudim Usfas HaSlavim, see page 20 here where he mentions that Rashi.
The point of HaYehudim Usfas HaSlavim was to address the Rishonim mentioning Slavic words. He doesn't seem to focus on why they would call the Slavic language(s) by the name Canaan, unless I missed something.
Is it related to slav = slave = eved knani? Is it a general term for non-jews (if so why specifically slavs?) Some other reason?
