There are many ways to look at which might be the most useful languages to learn in an immersion context. Are they the languages most spoken in the world, the ones most spoken in the US, those most commonly used in business around the world, those most commonly used in trade with the US, the ones which allow us to study the past, or the ones that allow us to travel the furthest?
The good news is that being immersed in a foreign language during early childhood does what it does to the brain regardless of which language it is. Bring on Latvian!