Summary
#1: Probably one of the most popular APIs is Google Translate. I never knew what APIs were, so I wouldn't know about the popular APIs, but anything by Google seems like it would be, especially Translate. Anyways, Google Translate contains over 100 languages that it can translate into, and the advanced plan includes more features like customization support and document translation. Both plans appear to be paid, but there is a free trial, and apparently new customers also get $300 in free credits for using the API.
#2: Unfortunately, this API is shutting down at the end of the month due to lack of support, but I wanted to include it anyways since it seems super useful: REST Countries. It provides a lot of information about any country like population, currencies, and languages. As for price, it does appear to be completely free.
#3: This one is an amazing collection of everything from one of my favorite video games: PokeApi. It contains all the information about every Pokemon from every generation including obscure info like what index number it has in the Pokedex in each game it's in. Also, inventorty items are apparently included. Finally, I think that this one is completely free like the last one.