I think picking any one existing language would be too emotional for people.

I don't have the implementation details. But just to illustrate how doable it'd be, I'd suggest we take something neutral (like Esperanto) and every language start adding 20 Esperanto words to their language per year, with a plan to coalesce after 200 years.

40,000 shared words isn't a full language, but it's enough to make conversations and debates possible.