dropdown for languages should present untranslated languages only

I encountered the following behaviour, that I consider a bug/misbehaviour:
How to reproduce:
Create a a new page, say node 23, language 'en'
Create a translation for that page, e.g. in 'de' -> node 24
Edit node 24 and change the language to 'en'
You get an error: user warning:

user warning: Duplicate entry 'en-24' for key 2 query: UPDATE localizernode SET pid=24, language='en' WHERE nid=24 in E:\xampp\htdocs\drupal\includes\database.mysql.inc on line 172.

This error could be avoided if the dropdown menu for the available lists presented languages that are not translated yet only.
Note that I encounter the same error frequently while using the 5.x-1.x version of the localizer module.
Were nice if we could get rid of this error in the new 5.3 version.