Just a link collection to prepare a feature request. Feel free to add and comment.

en.wikipedia.org/wiki/XLIFF

www.ibm.com/developerworks/xml/library/x-localis2/

translate.sourceforge.net/wiki/toolkit/xliff2po

Multilingual Web Applications with Open Source Systems
buytaert.net/files/gabor-hojtsy-thesis.pdf

For those who are not so familiar with translation formats, can you sum up how Limesurvey would benefit from this and how complex you consider the implementation?

Please post the link to the feature request here afterwards.

XLIFF is allowing you to exchange data with professional translators.
More and more tools these translators are using support this standard.

In the last years surveytools began to support this standard too.
Just export the file for e.g. english. Sent it to the translator and getting back the language file to import it back.

I have included a few links for developers to get a quick impression.
E.g a toolkit to get xliff together with gettext/po to maintain parsing speed.

From what I can judge so far, using the XLIFF-PO tool should do the trick for Limesurvey?

Or are you thinking about a more integrated solution? How could this look like?

We have the backend/frontend and the survey itself which demands translation.

There are many ways to work with translators on surveys.
One would be a translator mode in limesurvey. Similar to a testing modus. The translator is doing the survey like a respondent and sees e.g.the English version and fills in the translation in formelements on the screen. Often it is the test mode where on every page a box for comments is shown which will be shown in editor as feedback for the survey editor. Since limesurvey haven't got a test mode with skip and feedback form to integrate external feedback another way
would be a import/export xliff for the survey and the backend stuff.

The difference in the architecture would be that every survey project can has its own language-files.

But XLIFF can also be used to broaden access to language translations as just another way to edit po files. PO Editors vs. XLIFF Editors.

CAT (Computer Aided Translation) Tools will sometimes support PO too.
So the question is: Is there a demand for export/import of languages-files for the back-end and the survey via the gui?

jelo wrote: Hard to say. There are many people who use multilingual surveys but I haven't heard that lots of them need some XLIFF standard.

Just create a feature request and we'll see how many people wil vote for it.

Too early. The feature request section is seldom visited.