Yiddish used by haredi (in Israel at least) is more and more based on Hebrew (definitely phonetics). It's not the same Yiddish my grandparents spoke (Odessa and Uman regions of modern Ukraine).
It's also got a lot of English in it nowadays too, and Dutch. It's a diasporic fusion language so it sucks in the languages of its environment, which for today's speakers means Israel, US/UK, Holland (Antwerp).
Belgium*
Woops, thank you.
Looks like it's too late for me to correct, apologies.
my polish born and american (but "lithuanian" rooted) grandmothers would speak to each other in yiddish, but wouldn't always understand each other.