BwTransifex is tested on several Joomla!® installations without problems. Most notably I test with the most current versions of Joomla!®, which Joomla!® supports. Nevertheless I can't exclude that something goes wrong during installation. Especially the „interplay“ of different extensions can't be estimated fully and negative effects can't be excluded.
Therefore:
I suggest to always backup the complete system (database and files) before you make changes to your Joomla!® installation (installation or update of an extension)! For this task the free version of Akeeba Backup e.g. is suitable.
Testing an unknown extension – this is also true for BwTransifex – it is highly advised not to do at a life-system.