Just "Copy" the VTMB folder, create a new File and paste it on it, when you install multiples modes, send it to the new File where's the fresh copy, it worked for me in Half Life 2.
What i'm gonna type now I don't think this has something to do with VTMB but may also help who has similar problems like this, when Half Life 2 was released, it hadn't reached my country yet, so a friend ordered it by internet and I would pay him later, problem was, the game came on Russian lol (as most Valve releases), and was kinda "bugged", I was without net at the time, so no Steam support or anything, was quite simple to fix, for the bugs I made a copy of my original Half Life 2 folder and paste it with my HL EP 1 files, after replacing it all, the bugs were fixed.Now the next step in change the language problem, for those that cannot acess internet when having this problem, go to the Steam folder inside the game on the Resource folder, inside there will be "platform_country) >>there are for many countries " and also vgui_country. And if you look inside the game folder also, there will be the Resource part, but there will also be : closecaption_country(in DVD and txt format),gameui_country, gametitle(depending on game name)_country and valve_country... what you do now is, when it's in Russian, he will be AIMing the game language to the files valve_russian, gameui_russian,etc you got to only put a tricky on the system, these that are on russian you put an "_" on the beggining so will be: _valve_russian, then the english file that will be valve_english you rename to valve_russian and do same to the others I put up there, everything shall be fixed.
I had to put this here 'cause someone may have these problems even on VTMB that have these folders.