The language is primarily fetched from the URL (and writes a cookie with that information). If non is present, that frontend_lang cookie is used (and added back into the URL). If neither is present a fallback language will be used (not lang key is added to the URL due to the default language being used).

lang key in URL:

Private browsing, no lang key in URL:

Now, what usually happens, pages indexed by search engines, have carry a lang key - and more often than not are indexed for multiple languages:

Thus, basically, whenever you navigate to odoo.com via a search result it's somewhat a gamble of what language key you will end up with. Clicking a forum-mail link then, since it does not carry any language information in the URL, will have you end up with the language of your frontend_lang cookie (the language of your most recent odoo.com visit).

I think it has something to do with whom response to your message and it takes their language settings, can anyone confirm this?