When you start the software using the Revit shortcut, it opens using the most recently used language pack, or the core software, if that was used last.
If you want to always launch the software in a specific language, modify the target line of the Revit launch icon, as follows.
Example:
"C:\Program Files\Autodesk\Revit 2014\Revit.exe" /language FRA
The 3-letter language codes are as follows: