If the 'format' and the 'strftime' happen simultaneously, then 'format'
will use the "foo" locale, which is incorrect.
During 'with-locale', we would have to prevent other threads from doing
_any_ locale-dependent operation. In a system like Guile, where a
process may contain arbitrary C code and shared libraries, there's no
sane way for us to do this. In any case, it would not scale well.