Has your system language changed? Something has to be telling it to load Spanish…
Nope. I’ve made no changes to anything, apart from setting up a new profile and then switching back again. I couldn’t say for certain when the language changed for add-ons. It was definitely in English the last time I looked - which may have been a week or so ago, I really can’t remember.
Is it time to tread carefully into about:config ??
Possibly - although I’m beginning to suspect the issue may lie with Mozilla. When I followed the “la misión de Mozilla!” link in the top right-hand corner of that image, it took me to a page in - German.
Edit: I’ve just checked on my laptop, and the add-ons menu is in Spanish there, too, so definitely something strange at Mozilla. (From my laptop, the link goes to an English-language version of the page, not German.)
I’ve never noticed this 
…looks interesting though 