The last couple of beta versions (2.1.5, 2.1.4, 2.1.3) the UI language reset itself after I updated SourceTree. I use Windows 10 with a german locale and SourceTree is set to english.

After a beta update has been downloaded and SourceTree is restarted the UI is in german, even though in the config dialog it says the language is english.

To set the language back to english it is enough to change the language dropbox from "english" to "automatic", and then back to "english". After a restart SourceTree is in english again.