Replace Text Multiple - but translate only once

Hi question,

I am searching for a python script that can translate multiple text fragments.
On 1 condition: when a text part is translated, it should NOT be translated again..

a =

for item in T:
for i in xrange(len(F)):
fragment = F[i]
replaced = R[i]

    item = item.replace(fragment, replaced)

a.append( item )

Any suggestions?
Thanks!