I modified the prtDefault.properties and the prtCentral.properties.bak by adding:
request.mandatoryLanguage=en
request.mandatoryCountry=us
in order to force the language of the portal to EN. This did the trick for the logon page, but I am still seeing the locale of the browser, once I have logged on.
<b>Did anyone come upon this and manage to fix it?</b>