Check the bottom right corner of this website.

says english -the point?

If you came to the site through an article on a search engine, it might have had a non-English locale code in the URL. Could you check the first part after the host name in the URL you're visiting:

https://support.mozilla.org/en-US/

https://support.mozilla.org/en-US/questions/1499173#answer-1718048

again, your point?

This is really strange. I can't reproduce it. Moreover, I know Spanish a little and that's not Spanish nor Catalan. Something like Portuguese, I really don't know from where it's coming from.

It's not form us for sure.

https://support.mozilla.org/en-US/questions/1499173#answer-1718048 again, your point?

Thank you for checking that, and apologies for not being clear: by "the URL you're visiting" I meant the page in your screenshot.

Some of the phrases in the screenshot match with the localization for Galician, such as "Ferramentas de edición" for "Editing Tools" and "Como colaborar" for "Continue" (Pontoon). That localization appears to be incomplete, so it has the odd mix of language in your screenshot:

https://support.mozilla.org/gl/kb/frequently-asked-questions

The language code in the box toward the bottom of the page should say "Galego" on that page. When you are accessing the page using the en-US locale code, you shouldn't see the Galician bits:

https://support.mozilla.org/en-US/kb/frequently-asked-questions

Here is a url with the errant(?) coding: https://support.mozilla.org/gl/questions/1499119 gl=good luck?

I may be able to offer the offending tar - would it be of any use? and is there anywhere I can get docs on how to navigate this website? it seems to need a better threading tool

There is no threading, but you can quote by using the menu to the right of the poster's name.

Maybe you can use your history to track back how you got into the "gl" locale in the first place.

gl is the abbreviation for Galego as jscher2000 mentioned.

For example if you go to https://support.mozilla.org/en-US/gallery/images and click on Show media for: you should see Galego (gl) in the list of locales for this site.

If you find Knowledge base articles or threads here by way of search engine search (instead of using search on this site) you can sometimes get a KB or thread link search result that is not en-US. You can manually change it to en-US or you can even remove the locale in the link and it should default to en-US when you load it.

For example if you remove the gl from https://support.mozilla.org/gl/questions/1499119 as https://support.mozilla.org/questions/1499119 then it should load as https://support.mozilla.org/en-US/questions/1499119