i can change the files, but what will be the matter with multiplayers.
As far as I can tell without going into technical realms that I have no knowledge of, computers use checksums to determine whether two programs are identical/behaving identically.
<speculation>
I infer that if you change the files the provide voices and language, your Outpost 2 will end up with a checksum different than that of the other player's Outpost 2 programs, indicating to the other player's computers that you are running an irregular version of OP2.
The other OP2s will then refuse to work with yours.
I imagine this is either: A) A precaution to prevent incompatible OP2 versions from trying to play with each other (which could result in a whole variety of errors), or B ) A precaution to prevent cheaters/hackers from giving themselves an advantage by altering their game.
Based on other people's comments, I gather there is some way around this, but I'm not versed in the ways of multiplayer, so I don't know what that method is.
</speculation>
My advice: Make a backup copy of your OP2 folder on your hard drive (simple copy/paste), and then make any changes you want. If it turns out you ruined something, you can always restore the backup.