WordPress has no ability to switch language based on URL or query string parameter. In fact it does no dynamic language switching at all. Wherever you've seen this working, it would have been using a ...

I downloaded the free Rookie parent theme and have no problem displaying translations of "Search Results for: %s" on a successful search results screen. This works in the WordPress languages folder ...

You can ship translations of some of the metadata used in your main plugin file's header comment fields. Namely: Name, Description, PluginURI, Author, and AuthorURI. Just add the source strings into ...

Don't write your own bootstrapper, use Ajax hooks via wp-admin/admin-ajax.php Documentation: https://codex.wordpress.org/AJAX_in_Plugins Then you can be sure WordPress has included all the functions ...