Actually, the coding guidelines that we followed originally state:

"Static Variables 
Static variables may be prepended with 's'. "

We use $s_ for static elsewhere in the code base.

Given that we have discussed changing languages files to use either gettext or an array format in the future, and that we use single characters to define variable names outside of classes in all cases, I'd probably go with $s_ should be static, and if we want to do anything, we should change the language files to use $l_ (for lang). 

Personally, i'd use $s_ and leave the language files as is (also using $s_) as being pedantic here, the language files have a .txt extension so could fall outside our normal coding guidelines and still use $s_