What is "clear and understandable" varies greatly, though. E.g. Nigerian English is often subtitled in the UK, but fairly often so is Scottish English... Both o

What is "clear and understandable" varies greatly, though. E.g. Nigerian English is often subtitled in the UK, but fairly often so is Scottish English... Both often to the great dismay of speakers of the two who sometimes are very annoyed at the expectation that people might not understand them. Nigerian English is actually fascinating in that there's a whole spectrum from Nigerian Pidgin, which ranges from nearly unintelligible to English speakers, to "mostly British English" in terms of orthography and grammar, but which still tends to incorporate words from several differences Nigerian languages and pidgin. (e.g. abeg, don't give me any wahala; Please, don't give me any trouble) Now consider Nigeria is about to become the country with the second largest number of English speakers worldwide (it's close to tied with India, depending which sources and level of proficiency you consider, and Nigeria's population is growing far faster than India's), and while it's still quite far behind the UK for people speaking it as their first language, with current population growth and increasing use of English (e.g. my ex wife's first language is English because her parents first languages were Igbo and Yoruba, and that kind of situation is driving adoption) likely to cause Nigeria to become the second largest on that measure as well. So handling a broader range of dialects will matter, at least in terms of recognition - I do agree that there's more flexibility for generation, though even there if you try feed a broader Nigerian English pidgin to a TTS engine and it doesn't know what to do with the words it might well end up being unintelligible both to eg. American or British English speakers and Nigerian English speakers.