Support internationalization & localization in Qt Help

As a developer using the Qt Help Framework, I want to present my users with a localized version of my help documentation/user manual.

Localization is already available for other parts of the application via the existing workflow based on Qt Linguist.

However, help documentation is different as it is written in HTML and I cannot extract strings with lupdate, the way I can for other file types: https://doc.qt.io/qt-6/linguist-lupdate.html

I can use the QHelpEngine with identifiers to provide localized documentation, but the lack of integration with lupdate is a big issue as I have to manually keep track of entire HTML pages. This is also undocumented in the Qt Help tutorial.

My proposed solution would be to support translatable strings in the HTML documentation, for example via a HTML class attribute, that lupdate can extract and provide for translation. Other strings in the help files such as Tables of contents and keywords should also be extractable.