I found out when this happens.
Download this file
http://www.auby.no/files/video_tests/xvid_480p_as_l5_1mbps_he-aac_foreign_subs_matrix.mkvWhen I import the above file in PVD, Enlish Language is correctly Detected
Remuxed the above file using the latest mkvmerge without changing anything:
http://www.videohelp.com/download/mkvtoolnix-unicode-6.4.1-setup.exeWhen I import the newly remuxed file English language is not detected in PVD.
I then decided to use Header Editor to see if there are any differences between the two files and noticed that in the second file the "eng" element is not THERE !!
Google it and came up with this:
https://trac.bunkus.org/ticket/524Where it states that :
"Actually it is none of your three possibilities. The matroska specs have known "default values" for some of their elements since the specs were created in 2002. This is nothing new. For example the "track language" has a default value of "eng". A player has to use this default value if the element is not present in the file.
What changed in Mkvmerge is that elements whose actual value equals their default value are not written to the file anymore. This affects elements like "track language" or "default track flag". It is 100% spec compliant. It is nothing new. It is completely backwards compatible.
The only ones at fault are some players that do not implement certain parts of the.matroska specs. Most of the software implementations have been fixed in the meantime. For MPC HC there is a fixed version available if I am not mistaken."
So I guess that if English is considered default value according to matroska specs then PVD should handle the absence of the eng element as "English"