How To add translations to Qt Creator

Coordinate over the mailing list to avoid duplicate work.
Read the instructions at http://wiki.qt.io/Qt_Localization

Preparation

It is preferred that you work on the current stable branch. That usually is the branch with the same major and minor version number as the most recent stable release (not beta or RC) of Qt Creator.
Configure a Qt Creator build directory with CMake.

Adding a new language

Create a new translation file by running

cmake --build . -target ts_untranslated

in the build directory. This generates a file

  share/qtcreator/translations/qtcreator_untranslated.ts

in the source directory.

Rename the file qtcreator_untranslated.ts to qtcreator_<lang>.ts. (Where <lang> is the language specifier for the language you add. Don't qualify it with a country unless it is reasonable to expect country-specific variants.)
Add <lang> to the set(languages ... line in CMakeLists.txt in this directory.

Updating translations

Run

cmake --build . --target ts_

in the build directory. This updates the .ts file for the language <lang> in the source directory with the new and changed translatable strings.

It includes entries for obsolete (= removed) items and the source locations of the translatable strings. These can be useful during the translation process, but need to be removed before creating the commit (see below). If you do not need or want this information during the translation process, you can also directly run

  cmake --build . --target ts_<lang>_cleaned

instead of the ts_<lang> target.

Start Qt Linguist and translate the strings.

Start with translations in contexts starting with "QtC::". These are the most essential ones.

Do not forget to "accept" finalized translations with the "Done" or "Done and Next" action in Linguist, to remove the untranslated state from translations before submitting.

Prepare for committing. You need to remove the obsolete items and location information before the file can be submitted to the source repository. Do this by running

cmake --build . --target ts__cleaned

in the build directory.

Create a commit:
- Discard the modifications to any .ts files except yours
- Create a commit with your file
- If you added a new language, amend the commit with the modified CMakeLists.txt file
Follow http://wiki.qt.io/Qt_Contribution_Guidelines to post the change for review.

Note: .qm files are generated as part of the regular build. They are not submitted to the repository.

Note: QmlDesigner contains code from the Gradient Editor of Qt Designer. If an official translation of Qt for your language exists, you can re-use the translation of those messages by merging Qt Creator's and Qt Designer's translation using lconvert:

lconvert qtcreator_<LANG>.ts $QTDIR/translations/designer_<LANG>.ts > temp.ts

Move the temporary file back to qtcreator_<LANG>.ts, complete the Gradient Editor's translations and update the file, passing the additional option -noobsolete to lupdate (by temporarily modifying <qtcreator>/cmake/QtCreatorTranslations.cmake). This will remove the now redundant messages originating from Qt Designer.