ok i tested the exact way you isntalled , and i think i know whats wrong ,
1) DID you replace the -GUNDAM_MOD3.1CN_English.big in teh orignal
D:\GAMES\Homeworld2\Data (my isntallation path yours might be diffent)
2 ) WITH the GUNDAM_MOD3.1CN_English.big in the file D:\GAMES\Homeworld2\Data\English英语 ???
3) and use the GUNDAM_MOD3.1 batch file in D:\GAMES\Homeworld2\Bin\Release to start the mod
(yyou can make a short cut to teh short cut in your desktop)
4) if you do all thius it should come out in english even in gundam mod 3.1 , i tested it myself XD
PS: i think ytour problem is you forgot to replace the engish file cause you saw its alaredy in english ? (the title , but inside its chinese)