If you are using a server linux, you can find by terminal administrstion, editing directly on php application file.
First search the sentence using this command
Langdir of moodle application directory
Sebtence who you need to change
# find /langdir | xargs grep -s -a -i sentence | cut -d : -f 1 | uniq
The results are the files wich has the sentence, just edit then
!! Be careful!! Its for advanced users