Create an interface that presents the welcome text, groups, question text, help text, sub-questions, etc. to the translator, and then writes the translated text directly to the database.
You can view a demonstration here:
http://surveys.pentalibra.com/translator/starttrans.php?sid=29846
There are two ways of approaching a solution.
1) Present the translator in an interface outside of LimeSurvey admin. In other words, the idea is similar to filling out a survey. The presenation can be customised using a CSS template.
2) Present the translation interface as part of the LimeSurvey admin interface. This will require creating a new user type of translator, and will require login.