polylang-pro

With over 500,000 installs, Polylang is the most popular multilingual plugin available on the WordPress directory. You write your posts, pages and create categories and post tags as usual, and assign a language to each of them. Maintained by wordpress-premium on GitHub, where it has earned 54 stars from the community.

It's actively developed around translation, wordpress, wordpress-plugin, and is a solid reference for anyone building with these tools.