Removed BOM from 3 language XML files

We have removed the BOM (Byte Order Mark) from the beginning of 3 of our language XML files (en.xml, de.xml and fr.xml). The es.xml file did not have it.

This change fixes a very rare issue where the xml file would trigger a parse error in more strict XML parsing environments. We’ve seen the issue with an Ionic/Capacitor app trying to parse our en.xml file.

Custom language docs have also been updated with BOM and Content-type header recommendations.