Elves who don't speak Westron

[akhtene]

Nice question, Kalimak

From what I read in the Appendix, Westron was in origin the language of the Atani and probably suffered much Elven influence. Furthermore, Numenorians returned ro using their ancestrial language after ages of speaking Elven, so most probably they borrowed a lot from it into their tongue. Thus I'm trying to say that Westron couldn't have been absolutely alien to elves.

As for changes in the language, they don't happen in a day. Those who (like Elrond) were using it for regular communication must have had chance and time to update their vocabulary and what else you need to understand another. Only if you stay cut off the rest of the world, you may start facing the fact that your language is outdated. And becides, the motivation to learn or keep up a language disappears.
Perhaps JRRT had some other explanation to this. For most part those are just my ideas brought up by learning and teaching a language. And by the 3rd night at the screen...