JSF: Inconsistent browser behaviour when setting <language-select> to false

[ Steps to reproduce ]

1. Add <language-select>false</language-select> to the web-client-config-custom.xml file as:

<config>
<client>
<language-select>false</language-select>
</client>
</config>

2. Set the browser locale to Spanish

[ Actual Result ]
Initial user name / password scren shows in spanish, then once logged in, UI is in English

[ Expected Result ]

UI to be in Spanish

[ Notes ]

In-house:

On 4.1.5 and 4.2:
Firefox 25.1 (on windows, works)
Firefox 25.1 (on ubbuntu) fails but somehow when using the server's IP instead of localhost, it worked.
Firefox 10 (on mac works).

Customer:
Fails on Firefox v.24 or IE 10.