Well,,,
Video Part
At first convert the .bik file to mp4..
Then use handbrake.
Convert with xvid codec and then again bink it.
Language part
As we already know gta game is easy to mod. Then He may have opened the game folder and ripped some language and also you can crate dummies of the other language. Some game backup companies like corepack and dz team did it to 30gb. :P
So try to rip some language.
Audio Part
Then you can open game content and convert audioes to 96kbps bitrate
Note: AS for GTA 4 some russian people ripped all game file and done modification in texture as a result the game was 8gb from 15gb :0 Then they used precomp+srep+rep+lzma and it worked fine but the game used to lag a lot and used to consume much resource