wrong lang prefix makes the operation fail

[dontbugme]

Trying to extract french subtitles to srt from idx/sub than contains english, spanish and french, I installed the french localization for tesseract but vobsub2srt conversion still fails because it's looking for files prefixed fre. when the installed files are prefixed with fra.

for example vobsub2srt requires fre.unicharset when the french tesseract package installed fra.unicharset

[tomtor]

See my issue report, it's easy to fix. Change fre to fra in langcodes.c++

[ruediger]

See Issue #9