U4-2449 Feature request: Change the default admin users lang to en-us

Changed the default created language during install from en-US to en-GB, which is also the default language of the admin user created during install. This eliminates confusion when people are trying to translate the backoffice (documenttypes, descriptions etc.), since they would first need to add the english language before the stuff works. and is simply just much more consistent.

Made the change for 7.1.0 but it should probably also make it into the release of 6.2.x :)