I'm having a problem with localization.
A website of mine is available in 4 languages: nl (default), fr, en and es. Everything works fine except for the localization in English. It's doesn't translate the localized string in English (en_US.mo) but instead uses the default Dutch. The other languages' .mo files work just fine.
To create the theme's .po and .mo files I used Codestyling Localization plugin. I pulled the en_US.mo file from the server and tried it on another server and it worked just fine. Same for the entire theme. So, I'm thinking the problem must be elsewhere.
Other things I tried are: deleting and recreating the english .po and .mo files, removing the Codestyling Localization plugin and the theme language folder, reinstalling the plugin and recreating.
I'm at a loss. Currently, I'm thinking a the client might have done something wrong while creating the language (through polylang) and that is causing the problem. How could I verify this?
Do you have any other ideas what might be causing this?