I would choose to master complete linguistic skill, thereby being able to learn any language almost immediately and speak/read it as well as any native speaker. This skill would be awesome for a few reasons, the first being that I’d never have any trouble travelling.
Lost in the Australian outback? Let’s pull over and ask for directions.
No, the nice German man was not threatening to kill you; he was complimenting your shoes.
Time Lord lands in ancient Rome? Ave, Imperator! Morituri te salutant! Wait…
I would become a tremendously valuable asset the world over as a translator and since my understanding of both languages would be perfect, there would be no “lost in translation” moments. I think treaties, negotiations, and summits would benefit, certainly.
The downside would be that I might be constantly in danger because sometimes people just can’t abide a solution. Or I’d be seen as a threat because government X was counting on insulting government Y and starting a war for nefarious reasons.
Still, being able to travel at will and speak like a native speaker would definitely be an asset, especially since if I am looked upon as a threat, I can just blend into all the communities.