Language translation
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A good place to start is "here.":http://qt-project.org/doc/qt-4.8/qdeclarativei18n.html And "here.":http://qt-project.org/doc/qt-4.8/internationalization.html
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Not forgetting "bug":https://bugreports.qt-project.org/browse/QTBUG-15602 which means that QML doesn't handle true dynamic translations well. There are some work arounds but they're not very pretty.
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Here is one of them:
"workaround":http://qt-project.org/wiki/How_to_do_dynamic_translation_in_QML -
The links that @Rocken and @mlong gave have pretty complete examples of what you need to do. I would suggest reading those and following links through the wiki / help to find out what you need.
You can do most of what you need in QML, but the functionality to load and set the translators will need to be in C++. See the documentation for "QTranslator":http://qt-project.org/doc/qt-4.8/qtranslator.html for help on this.