I'm sorry, what is the question? GitHub renders in your preferred language? Or doesn't … or you have set something completely else and it still does? You're logged out, no account, no session — no prior cookies et al., never authenticated to GitHub using their MFA that requires a mobile phone number etc., and have setup as to not "appear" as coming from some region, but GitHub still places you in the region you don't want to appear as coming from?

That webpage redirects me to a specific language option URL query. Browsing to root URL, in any condition it redirects to `github.com/?locale=es-sp` or any locale code which should remain in default language even the history or data of that page manually removed.

I had to turn `geo.enabled` to false manually via `about:config` to avoid sharing my location always until I prefer to do, and sacrificed all my sessions by wiping all history. Also had to install extra privacy extensions which I wouldn't need.

My question is, how Firefox, as default, can't avoid this while there's no specific personal setup for the website?

intl.accept_languages

What does https://echo.opera.com/ say for your content negotiation headers? (See the image.)

If you have es-ES as your preference there, it will be the reason as to why.

This is the long read: https://www.w3.org/International/questions/qa-lang-priorities/1000

Here's a quick test: https://manytools.org/http-html-text/browser-language/

This is the preference you might need to adjust to your liking if the above shows it's the case: Choose display languages for multilingual web pages

(i.e. this has nothing to do with geolocation (!), but content preference — that might come from your installed language variant or underlying OS setting in the default state.)