When you say "install new language", you mean from "Extensions" > "Language(s)" > "Install language" button, right?

Since my intranet websites can't connect to internet, I install language packages using "Extensions" > "Manage" > "Install" > Upload Package File.

I have just tested here (Joomla 3.4.8) and this issue didn't happen. I don't need even to logout and login again to have my Control Panel in the new language. It is automatic for me, in the moment that I set the new one. If I logout and login, I still see the Control Panel in the new language.

When logout/login and the user preferred administrator language is NOT set to default or NOT to the newly installed language but to another language, it is this other language that will be used. In your case en-GB.

@anabarcellos Yes, "Install language" button.
@infograf768 My user settings are all set to default.

I can't reproduce your issue.

Have you deleted all Caches? Joomla and Browser?

@infograf768 Yes, I have deleted all caches. Didn't help. Only way I could get desired language was to select it on login. Even setting it under user profile didn't help.

Could it be somehow from server configuration? Where is language set on source? Can't check source right now. Also, all other clients sites running on our company server and on 3.4.8 (upgraded) works fine. Only this has problems (client server, fresh 3.4.8 install).

@infograf768

I investigated this issue and found out that client server is running with Suhosin PHP extension with suhosin.post.disallow_nul = 1 which results empty $_POST variables to be removed. So there is no lang key in $_POST.

Therefore on login, settings language will always default to en-GB in here.

$lang = $this->input->getCmd('lang', 'en-GB');

Solution would be to remove 'en-GB' and if it's empty, it would use default language like expected.

Set to "closed" on behalf of @infograf768 by The JTracker Application at issues.joomla.org/joomla-cms/8906

Closing as we have a patch #8956