Yes if you were to stall the airplane while in a side slip it will probably depart, or start to depart, into a spin. How aggressively would depend on the glider's actual stall behavior, some being more benign than others, but say you were in a glider with an abrupt stall with minimal warning and a sharp nose drop. That sort of glider would spin if you didn't initiate recovery right away.
This isn't such a big problem, because the nice thing about gliders is you don't have power to confuse things when it comes to pitch attitude vs speed. For a given configuration (weight/CG etc), if attitude X gives speed Y than you can be certain that any time you pitch to attitude to X you will get speed Y, with or without an airspeed indicator.
So, if you are on final at 50 kt, and want to side slip, just make sure you lower the nose a couple of degrees during the slip and control the pitch attitude, so that you will maintain somewhere around 50kt with the additional drag and there is very little risk of stalling and spinning. Easy peasy.
You also have the other impending stall cues, like softening controls, noise dying off, and such, so if you are aware and on top of things there shouldn't be a problem and it would take some ham fisted flying to really mess up.
If you are getting trained well, there should be dedicated no-airspeed instruction and practice and once you are considered a proficient pilot you should have no problem with flying safely with no airspeed for all maneuvers (if you sent me out in a power plane on some kind of wacky challenge, and told me I could only have one operating flight instrument, my choice would be a compass).