I agree it would be cool if there was a good IPA speech synthesizer, that would certainly help! There's some discussion on the issues and problems with it here: https://news.ycombinator.com/item?id=13514258
If you take a random language and believe that you can pronounce it correctly by reading IPA, well that won't work. Or at least be very hard.
Because English is spelled non-phonetically, English learners across the world generally learn IPA together with English, to learn pronunciation.
Granted it's not the entire set of IPA sounds, and some students are better than others... but seeing as there many more speakers of English as a second language than native speakers (according to some sources, though this can be disputed), there must be at least hundreds of millions of people with a rudimentary knowledge of IPA, enough to sound out words in English.
The actual sound that gets produced can vary by context, what sound you made before that and what will come after etc. (And who is saying it off course).
See this diagram: https://i.pinimg.com/originals/f8/1e/ea/f81eeaa4921133c3b67b...
As a local I hear (and use) all of them interchangeably and am only vaguely aware of the difference if I consciously think about it
What do foreign language textbooks in English-speaking countries use to teach pronunciation?
Using own systems I have seen in very naive tourist guides and in text books by English speaking publishers.
Honestly, we're all basically just hoping whoever wrote it knows what the symbols mean. An example: Greenwich (https://en.m.wikipedia.org/wiki/Greenwich).