[KevinH]

Do you set the proper language in Sigil for both the interface and the language? At the system level, do you properly set the locale environment variables?

Any double-click block highlighting is controlled by Qt not Sigil. I will search to see if there are any related Qt bugs.

Exactly what platform, version of Sigil, and version of Qt are you using (see the Sigil "About" dialog)?

Update:

Nevermind, this is expected behaviour in all Qt applications as Qt QTextEngine::atWordSeparator() is not Locale specific.

So double-clicking to highlight a word does not depend on language/locale in Qt.
If it matters to you, you whould probably submit an official bug report to Qt so that they fix this.