Today I have this problem that I could connect from a machine, running windows 7.
The error I got was:
The system cannot find the file specified.
C:\Windows\system32\<LANG_NAME>\mstsc.exe.MUI

I then search for mstsc.exe, and right click on it, and choice properties, and choice tab Details, look for language. In my example, my computer was a Danish computer, but MSTSC, was in English.
Look also for the version, if it 6.2 it is a 8.0 version, and if it 6.3 it’s a 8.1 version.
Then download the English version of mstsc (in the right version) on yours computer:
version 8: https://support.microsoft.com/en-us/kb/2592687
version 8.1: https://support.microsoft.com/en-us/kb/2923545 
And install it.