For now we will leave the locale switching in as a default in the development versions, document everyting well until more research is done.
In PCRE there are different issuee, I looked through the code an there is no locale switching. Everything seems to be dependant on character tables, which are generated before compiling. May be PCRE could be the reason not to automtically switch the locale but distribute newLISP with (set-locale 0 "C").
Unfortunately I don't know how find/replace/regex are performing i.e. on case specific stuff when the locale is switched.
Again, I think I have to do some reading first, to figure out how others solve these issues.