"Error" strings and slow down

[Mal]

I started up Alien Assault after quitting within a mission (specifically mission 2 from the Aliens mod) and every string in it was set to "Error" and the game also ran incredibly slow. I quitted out of the game and back into it and the large strings still displayed "Error" though the small one (campaign progress) showed the campaign name and progress just with error written at the start of the string).

[the_Fifth_Horseman]

Replace all language.txt files.
If that doesn't help, delete the options file.

[Niklas]

Sounds very much like a data error on your side and not a game bug since no one else have ever noticed this while quitting in a mission and rejoining.