optimize pre-translate script by compiling regexps

Pre-translating regexps before hand outside the loops, brings speed up
of nearly 13%:

Before:

time scripts/pre-translate-po po/documentation.fr.po po/pre-translate-dictionary.fr.po
scripts/pre-translate-po po/documentation.fr.po   0,74s user 0,02s system 99% cp
u 0,760 total

After:

time scripts/pre-translate-po po/documentation.fr.po po/pre-translate-dictionary.fr.po
scripts/pre-translate-po po/documentation.fr.po   0,64s user 0,02s system 99% cp
u 0,659 total

Signed-off-by: Jean-Noël Avila <jn.avila@free.fr>