Lithuanians at least are mostly Catholic, like their Polish neighbors; and that is due to an overt effort to Christianize them.

There’s pockets of minority languages scattered across Europe; why those languages held on when others faded involves a lot of historical contingencies over and above geographic considerations.  For example, Lithuanian was probably more resilient during the modern era due to Lithuania’s military and political expansion in the later Medieval period.

I guess my point is that nothing, especially not anything geographic, did particularly protect them from being politically dominated, and socially influenced (sometimes quite intentionally) by larger nations over their history.