Thanks a lot for your help. I tried to follow your recommendations, but I did not get rid of my problem. I kept trying, and I think i found it, at least for my case. But it can also help a lot of people with the same issue.
The thing is that the english text in the register.php file and the english text in the .po file do not match.
The register.php file says:
“Registering for this site is easy. Just fill in the fields below, and we’ll get a new account set up for you in no time.”
The text in the .po file says:
“Registering for this site is easy, just fill in the fields below and we’ll get a new account set up for you in no time.”
Once you edit the text in the register.php file (change the dot after the word “easy” for a coma and erase the coma after “below”) the translation with the .po file works perfect.
If anyone that take a look at this post can notify Buddypress people, or people responsable for the translations, to fix this little issue, will take away a lot of headaches of non english web devolepers and designers.