How to change the field used for the page names in the cms tree

if you are referring to the URL name (example /contact-us) this is the node alias path and there is one per node, as there is only one node that has multiple language variations.

You can define language aliases through properties > URL and also change it there.

What field that pulls from is in the page type (edit) > fields and the two drop down under the field lists, one for the node alias source and the other for the document name source (which can change per language)

These must be required text fields in order to select from them.

@trev - Sorry I didn't see your reply until @rene's today! What I mean is in the CMStree, the name shown to the user. I'd like ALL languages to show the English page name in the tree.

Ah i see now. Under the Page Types there is the a drop down "Page Name source field" set this to the field you want to show up on the tree (also controls the Document Name), it has to be a required text field. Then on your current language put in the name you want, should do the trick!