You have asked a very important quesiton.
I have overlooked the problem case.
In Pootle, by default, a new registered user is eligible to add new strings and _edit_ already translated strings.
So any person on the Internet is able to modify (spoil) the work of our translators.
I wish it could be possible to allow new user to post new submissions, but forbid editing of existing ones.
But I was not able to find this option in Pootle.
I think our best choice is to forbid strings submission by default for new users.
However, they can make string suggestions.
Superusers then will need only to submit or reject the suggested strings.
Later on we can give the submission rights to the user if we feel he is a trusted person.
I have added this text to the guide to make sure everybody respects the original translator work.
Before you begin translation, please remember this very important rule:
Feel free to submit new translations.
But do not modify submissions made by other translators.
If you feel the submission is incorrect, you need to add a suggestion.
Feel free to submit/discuss suggestions on our forum.
One more note about our translations security.
All the translations are backed up to my notebook every day unless I'm not working on the day.
In an unlikely case you to find any translations corrupted, please let me know.
I will restore the translations from the latest backup.
Werner, thank you for spotting the problem.
I have moved your post to "YoWindow.com website translation" forum since the question is about website translation. I hope you don't mind.