WordPress - Troubleshooting - I see a page not found (404 error) when changing language
In this article, you will find how to solve the 404 error problem when you try to translate.
If you have a page not found (404 error) when switching language, it's most certainly related to your Permalinks settings.
To fix it, follow these instructions:
1. Go to your WordPress admin, in Settings > Permalinks
2. Select anything but NOT the first choice ("plain") and make sure you don't have /index.php in your URLs.
3. Save your changes
4. Refresh your web page and it should be working
If the problem is not solved, check if the “mod_rewrite” module is enabled on your website. If not , you would need to enable it. To do so, you can either contact your hosting provider and ask them to enable it, or if you're managing your own server, follow the steps of this link: https://stackoverflow.com/questions/869092/how-to-enable-mod-rewrite-for-apache-2-2