You should also have an *.idx file as well the *.sub (the *.idx contains the subtitle timing/settings/language etc)
eg.
John Carter.mkv
John Carter.sub
John Carter.idx
Now, i don’t have any multi-language subtitled movies to test … but looking inside the John Carter.idx (use Notepad) you can see the language setting…
# VobSub index file, v7 (do not modify this line!)
size: 1920x1080
org: 0, 0
scale: 100%, 100%
alpha: 100%
smooth: OFF
fadein/out: 0, 0
align: OFF at LEFT TOP
time offset: 0
forced subs: OFF
palette: 000000, f0f0f0, cccccc, 999999, 3333fa, 1111bb, fa3333, bb1111, 33fa33, 11bb11, fafa33, bbbb11, fa33fa, bb11bb, 33fafa, 11bbbb
custom colors: OFF, tridx: 1000, colors: 000000, 444444, 888888, cccccc
id: en, index: 0
timestamp: 00:04:17:883, filepos: 000000000
timestamp: 00:10:23:164, filepos: 000001800
timestamp: 00:23:58:437, filepos: 000003000
timestamp: 00:24:05:361, filepos: 000004000
timestamp: 00:24:06:695, filepos: 000005800
timestamp: 00:24:12:493, filepos: 000006800
timestamp: 00:24:14:662, filepos: 000008000
timestamp: 00:24:16:997, filepos: 00000a000
timestamp: 00:24:33:180, filepos: 00000b000