Symptoms

1. Installed language pack in Confluence does not take effect when the global default language is set in Confluence Admin > Language for your Confluence installation.
2. Installed language pack in Confluence does not take effect when the default language under User > Settings > Language > Automatically detect browser setting is selected.

Cause

 Confluence does not respect language header from browser.

Resolution

Add the following line to your JAVA_OPTS:

-Dconfluence.browser.language.enabled=false