I thought about your question for quite a while. In my opinion in those cases where you can translate "zum" as "for" it is almost always possible to replace it with "für".
In your example that would be:
Anwendung einer Sprache ist wesentlich für den Spracherwerb.
This doesn't work however in other examples where you can't translate "zum" as "for" i.e.
Es ist zum Weinen.
For your second question you could use "anwenden", "verwenden" or "benutzen". However you could also use another sentence in German along the lines of "Um eine Sprache zu lernen muss man sie sprechen" which would sound less scientific.