Plone 4.3: changing personal language preference doesn't affect UI

Of course, you might have found a bug -- but I doubt it.

The browser language setting is not a simple list of languages but the languages can have an associated number which indicates its importance. The web site should choose the supported language with the highest importance.

My browser sends the following language preferences to web sites: de,en-US;q=0.7,en;q=0.3. This means de has importance 1; en-US has importance 0.7 and en has importance 0.3.

To view the information sent by the browser to my Zope(/Plone) instance, I have a "DTML method" (appropriately named show_request) with the following definition

<html>
  <head><title><dtml-var title_or_id></title>
  </head>
  <body bgcolor="#FFFFFF">
<dtml-var REQUEST>
  </body>
</html>

It shows the language preferences sent by the browser in the section environ under the variable HTTP_ACCEPT_LANGUAGE.

I've completely removed non-English languages from Chrome settings, then confirmed only en-US and en is detected online, but unfortunately Plone's UI is still show in Portuguese.
F

Then likely you somehow have screwed up your configuration. Plone definitely can present its UI in english -- something in your settings prevents it from doing so.

In such a situation, I would use debugging to find out what is going on. Another possibility could be to check the behaviour in a fresh Plone installation (one not yet screwed up).

Could it be the site top administrators have forbidden users from overriding the default language?