I recently came across a badly written mouse software that had an interesting issue. Whenever I clicked in a specific edit field it would switch to English keyboard layout even though none were configured.
The most common cause of the issue you describe is to accidentally press alt+shift to change the languages but that doesn't work in your case when only one language is configured.
I would try to pinpoint when it happens, is it in a specific application? maybe even a specific edit field or dialog box in an application? Also you should check for any keyboard software you have installed and try to disable or uninstall that and see if it resolves the issue.