I think I've found it.....
If you open a terminal in Slackware and enter:
what does it return?
As a second test, execute the same command from a terminal in VL 6.0 and let me know what it returns....
As a further test, open an Aterm terminal in VL, enter the following command:
Then start MC and see what it looks like.
I think the problem is down to the fact that Slackware has made no attempt whatever to get proper locale handling. You will probably find that the LANG variable in Slackware is either C or en_US. Aterm can handle those locales, where it can't handle something like en_GB.utf8 (which is what I use).
As a last ditch effort to get MC to be usable in Aterm, you could modify your script to include this:
before you start MC. That will problably fix it, though its a horrible kludge.
Another alternative would be to use urxvt. Unfortunately, I don't like the key mappings it uses (you can't use ALT-S to search in MC, for instance) and my attempts to fix the mapping haven't worked very well. I've also managed to break something in the setup that it uses, since it is now coming up with fonts so tiny that even with a magnifying glass, I can't read them (and I like
Anyhow, let me know how you get on with that.