In the 22nd Century, wouldn't Uhura have joined Starfleet for its merits of exploration and first contact scenarios, rather than its militaristic stance, and would be more open minded to alien languages and dialects?
No -- Hoshi did, but Uhura didn't join in the 22nd century

Most people - at least most educated people - are fluent in English and their native language. Some countries have multiple native languages.
If you learn multiple languages growing up, it's easier to pick up new languages as time goes on. Brits, Americans, etc have a bit of a problem with languages - what second language do you learn? Manderin? Japanese? Arabic? French? German? Spanish?
I've been to about 50 countries in the last 10 years, so I'm not exactly hiding under a rock internationally. I can count on 1 hand where French (the language I did at school) was spoken -- I think it's just France, Belgium and Switzerland (although the later two it's not exclusively French). German is just Germany and Austria. Arabic in a few more places (Egypt, Jordan, West Bank/Gaza, Dubai, Qatar), Mandarin, Urduo, Portugese, Swedish, Norwegian, Brazillian Portugese, Thai, Malaysian, Bhasa, Greek, Italian, Hungarian, and half a dozen more european languages. I can't possibly know them all, even if I was fluent in 5, 90% of the time I wouldn't be able to use it.
I ordered a coffee at Gar du Nord a couple of years ago, said "Une Café sil-vous-plait". Buy behind the counter responded with "three euros fifty". I'd just arrived on a train from Brussels too, and hadn't said a word of english since getting off the train.
What's the point in learning another language if you speak English? Chances are you won't get a chance to use it.