As I was installing an Alfresco on a Linux machine, I learned AGAIN.
Damn Alfresco, I know it since the "not yet 1.0" version (was 0.6??) and everytime I use it, I learn new stuff.

It all started with my test user seeing mixed languages, English and French in the web interface despite the choice of English at login time. 

OK, problem is solved but it was a pita as I had to "DROP DATABASE alfresco;". Was empty, fortunately.

Short version :
As the "system spaces" like "data dictionnary" are created upon first boot of the webapp, you must not leave YOUR language packs (.properties files) in the .war file. Main culprit was in alfresco.war\WEB-INF\classes\alfresco\messages\

Long version :
I installed on a Linux machine, not using the 1-click installer provided by Alfresco but it would not have changed a thing considering it's the .war file the guilty.

I am not a US resident, nor EN.
I use a Linux VM that I have not set up, physically located in fr_FR, which I tend to forget as I mostly write/code in english (sometimes frenglish).

On first start, the .war file deployed in Tomcat is unpacked and its content is used by the initial process -- including the deadly 

This process creates what I'll call the "system spaces" (like "data dictionnary") and they are created accordingly to the locale whenever the files are available. If not, the default are used and they are plain English, which is better for a system to be used by people around the world.

The culprit file can be found at alfresco.war\WEB-INF\classes\alfresco\messages\ I moved all files to a subfolder -- just in case.

My advice is to remove all I18N files from the .war before to use it.
Or, you'll have to drop the database and empty the alf_data folder and of course, bear with another looong first start. 

Hint: Setting the JVM's locale to "en_US" could be an option (not verified) at install time.