I don't think you can in the center pane. Right pane is controlled by the locale that you either set explicitly or is set by your system (or depends on the locale of Firefox you downloaded, if you're using Zotero in Firefox), which also controls the UI language.
Edit: I'll just note that we should probably improve this.
Thanks - I assumed this was inviolate (unless I changed code), but other day the date added in the central pane went to Euro date & thought - hey, good one! But no more ...
Edit: I'll just note that we should probably improve this.
Thanks
R