No. (I don't think changing general.useragent.locale affects the text of the Firefox UI itself if you're using a non-English version, but it would affect other Firefox add-ons and websites.)
Because I don't like the translation into Spanish. It also makes it much harder to google problems. Most answers are in English, so I first have to figure out what the instructions would be in Spanish. It's extremely annoying.
You can use Zotero Standalone in English, though.