Addressing just the primary question, I think there are clear examples where English-speaking mathematicians have been slow to catch up with developments well-known to German or Russian speakers. One I would mention is the result that surface mappings are generated by twists, published by Dehn in 1938, but rediscovered by Lickorish in 1963. The big advances in the theory of surface mappings due to Thurston in the 1970s were, I think, somewhat slowed by the fact that he had to rediscover many results of Nielsen published in German or Danish in the 1920s.

At a more trifling level, I was once embarrassed to learn that a result I published in 1987 was well-known to Russian mathematicians.