Variables VS constantes

Hello,
I would like to know if in a config file for
example, if it was better to use variables or
constantes ?
Is it a big difference of performance or in
memory used ?

And the same question about the translation of
a site.
I would like to translate my site in many
langages.
So I want to create one file per langage to put
in the text translated in it.
Is it better to use variables, associatives arrays
or constantes ?

Thanks a lot for your reply