There is TextConstants component, which provides pretranslated strings. (all default themes use it)

You can help us translate it to more languages, see this commit

There is support for theme-specific translations:

All strings should be wrapped with qsTr()

TranslationsDirectory property in metadata.desktop should be set. (eg. translations)

TranslationsDirectory should contain compiled(*.qm) translation files

*.qm files should be named as xx_YY.qm of xx.qm, where xx - language code, YY - country code

Tools to use: lupdate - to generate source(*.ts) translation files, lrelease - to generate compiled(*qm) translation files, linguist - to edit/compile source translation files.