From the FAQ:
Why does the language "eng" (English) not show up in the output file even though I've selected it in the GUI?
The Matroska specifications know a feature called "default values". These default values have been in place since the beginning in 2002. The meaning of the default value is that any application reading a Matroska file must use the default value if the element itself is not present in the file. For example, if the track headers for a track do not contain a "language" element then this means that the value "eng" must be used by the reading application.
Starting with version 4.0.0 mkvmerge does not write elements to output files whose value equals their default value. This mostly applies to the "language" and "default track flag" elements. If an application does not show "eng" as the track's language even though you've selected it in the GUI then please file a bug report for that application.
Some mkv players/media centers are unaware of what the default values are. Mine, for example, will display blanks for the language if I select the subtitle or audio track and the language element is not present in the mkv (as encoded by mkvmerge >= 4.0.0). I would like to request an option in mkvmerge (and mmg) that would force it to write the language element even if the default value ("eng") is selected.