AI Text-to-Speech models have accents too
blog.waleson.com
blog.waleson.com
This only works if your phonemes are encoded pretty generically, though. For example, /f/ in English is labiodental while it's bilabial in Spanish, so if you want your accent changing to work right you'll need to either represent both as /f/ or have a reasonable model for picking the closest sound a speaker of a given language is likely to be able to reproduce for any given input.
I think that the surprise disappears once we look at these two factors individually. As per jefftk's comment, it is to be expected that TTS in a certain language will be limited to the phones (and thus the pronunciation) of its language. On the other hand, intonation is always bound to sound "foreign" seeing as this TTS software cannot get even the original intonation right (try listening to the sample text with the US voice to see what I mean), let alone that of a different language.
And then there are other people in this thread who disagree and don't think most of them sound very much like a human speaker of non-English language speaking English! So maybe it's not obvious after all...
pico2wave also has:
German (de-DE) English, US (en-US) English, GB (en-GB) Spanish (es-ES) French (fr-FR) Italian (it-IT)
I think pico2wave's accents induced by cramming English text through the "wrong" language sound a bit better than the few I tried on the Mozilla web speech API, and it works offline, but I don't know that they sound good enough, similar enough to a real person's accent, to be really very useful for that.
For instance, "Can" is a Turkish name. So, the pronunciation has nothing to do with the accent in that case.
Some engines [ignore] foreign words, some pronounce as if TEE-AYCH-EE-EYE are initialisms, some are built multilingual or otherwise as flexible and accommodating as possible. OS included engines are flexible kind because users would make them say "Your Soufflé au Chocolat is arriving" et cetra.
e: ok maybe 100k or more than 10k
Edit: Worked on Firefox on Android