2019-11 We asked Polylang
The translations of your theme and your plugins are stored in the po.mo files. If the translations are not provided by your theme or your plugins you need to create them thanks to Loco Translate, they will be stored also in the po.mo files.
The content that you created yourself with Polylang is stored in the database as one post per language.
Feel free to comment if you can add help to this page or point out issues and solutions you have found. I do not provide support on this site, if you need help with a problem head over to stack overflow.