Yes, this solved it.
In polylang, I changed locale to en_GB.
Then I deleted the en_US.mo from my theme language folder and renamed the en_US.po to en_GB.po. Then I opened my codestyling localization plugin and created the en_GB.mo file. (little detour to keep edibility in the codestyling plugin on the english theme language file).
Thank you very much! Really appreciate your help.
As for,
That's probably the source of your issue. By default, WordPress supposes that everything is written in 'en_US' and it loads translations for other languages.
I'm not sure because I usually use a Dutch install of WordPress, so that should've had the default on nl_NL.
http://nl.wordpress.org/
But I could've 'accidently' used the US version as well. Can't remember.