When on Facebook the interface suddenly switches from English UK to a completely different language. No matter how many times I switch it back to English, Facebook reverts back.
Can anyone help me?
Following on from Hovsep's comment, web browsers can be configured to support various languages, and web sites (not only FB) will often provide pages in the first language on the list. If, for example, your browser is set to support Spanish, Japanese anb English, then the web site will attempt to serve the Spanish version if it has one, or Japanese, and only if it doesn't have either will it serve English.
Hovsep's response tells you how to diagnose the problem. If FB works fine on other browsers or computers, then the problem is in the configuration of your browser. Changing the list of supported languages varies from browser to browser so we would need to know the full details to give you instructions.
is it the same from another web browser, another pc?