Tried to look up the IPA for Tanwen for our pinned, hit a dead end. So I tried constructing it myself instead,1 and in the process discovered that whoever did Wiktionary's transcription of Taliesin is probably American, since they used an A sound not found in Welsh or British English. So I swapped that vowel out in our pinned, 'cause it seemed odd to transcribe Tali's name in an American accent.2
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I got /ˈtanwɛn/.
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I guess while I'm talking about it, I'll note that Taliesenn is not quite pronounced the same as Taliesin, on account of the swapped final vowel. Tali just used Wiktionary's transcription as a basis 'cause transcribing vowels is hard.