When adding a language, there seems to be a problem with case-sensitivity.

Example:

I added the Romanian language as a copy of English and gave it the International Language Code "RO" (with capitals). After this, neither the back-end nor the front-end would work, because it failed to open the file "lang-ro.php". The file however, was saved as "lang-RO.php" (with capital RO). Renaming the file manually using FTP did the job.

Great application though!