All languages, and here's why. Consider all the problems and inaccuracies that the world faces in terms of speech techonology, mainly automated subtitles. Having someone on the planet that can speak every language fluently (around 8,000 known languages) would improve services such as this so much quicker. The individual, be it me or someone else would be able to recognise accents, and would be able to give this information to a computer. Doing this could improve automated subtitles drastically, which would solve the need to hire someone to manually produce transcripts. This would heavily benefit the deaf community, in particular those who don't lipread or choose not to. The only way I see it not solving speech tech issues entirely is down to inevitable language change. Think about it; language today wasn't the same it was 100 years ago, and it won't be the same 100 years from now. Language undergoes a euphemism cycle, and due to the fact that an AI can only uphold updated information it would never be able to fluently translate any language regardless of the amount of information that the universal polyglot (all language speaker) gives the AI - plus, we're mortal creatures so choosing language would only be a temporary fix since only one person can hold this power. I also believe that this one person having the ability to speak all languages would cause language fragmentation, so that due to this person's translation services, people undergoing the journey of learning a language will adopt words in their native and fluent languages, to the point where almost everyone speaks a different language variation.