I tought it was a bugfix release

Hum, you should look closer. The version 5.5 is much more than a bugfix release! [:(] [:((]

Why its not possible to edit my own posts, at least for a few minutes ? Thats a security problem.

You're right, it's for security reasons but not the ones you had in mind. It's mainly to avoid spammers to publish nice messages then change them once moderated. Forums are nice but they require some moderation work as soon as they are successful.

why there is a difference between full installer an the portable version relating to the language translation ?

The portable version comes also with all the contributed translations (full or outdated).

But be patient, let's hope that more translation updates will arrive soon.