Qt Creator Translation Page
The authoritative resource about contributing translations is the Qt Localization page. This page is meant as a place to collect information on existing translation efforts. Please feel free to add information on the languages you want to work on and announce your intent at the Qt localization list. Some contact information would be great, too:-)
There is no English translation- it is the native language of Qt Creator. Regional variants and proper plurals are thinkable, but nobody cared enough to do a translation for just that so far.
The German translation is maintained as part of Qt Creator itself. Please check the Qt Creator Page for contact information
Japanese translation seems to be hosted here
See the blog post.
Active Russian, Chinese and Slovenian translation teams exist as well. TODO: elaborate on this.
User Interface Text Conventions
The rules are explained in the Qt Creator User Interface Text Guidelines.
As a developer, always be aware that an interpreter commissioned to create a translation of your code might have no knowledge of software development at all.