Hi Gerrit,
Stephen provided you with details for 3.0 SP1 and above and you are using 3.0. In SettingsResources.xml do you have a section called SAPLangKeys? Maybe if you search for EN you will find an entry. It should contain code like this:
<keys>
<key name="English">
<value>EN</value>
</key>
<key name="Chinese">
<value>1</value>
</key>
<key name="Japanese">
<value>JA</value>
</key>
</keys>
If you can find this, then change the <value>EN</value> for <key name="English"> to <value>FR</value>.
Best regards,
Luke