Also, note down the language ID of your language. This is useful when playing with the registry. You can find a handy list here (Look at the LCID Hex column):
1. Store your DLL (T9RomanianLdb.dll)
Place your language DLL in a folder under \Storage\Windows\
2. Edit the registry
Now, over to the registry goodness. There are two places you need to play with. The first one is:
HKEY_LOCAL_MACHINE\T9 Input Method\MUI\Languages
Click on the Available value and edit it. Add your language ID to the list and save. If you want to be extra fancy, place them in alphabetical order, as the list wont get sorted, it will just be read from beginning to end and your languages will be placed in that order.
Now, click back a couple of times and select the LDBs folder:
HKEY_LOCAL_MACHINE\T9 Input Method\LDBs
Here, add a New Value (String value) for each new language you are installing, the name of the Value is the Language ID and the Value Data is the path to the language file, for instance:
Value Data: \Storage\Windows\T9RomanianLdb.dll
Reset and go
Reset your phone for the above to take effect. You can now select your new language from the dropdown text type menu, for instance when writing a text message.