Hi,
unless I’m mistaken, the hyphenation works in Joplin.
I create a template containing
<head>
<style>
@import url("fullpathtoprofile/.config/joplin-desktop/for_tests.css")
</style>
</head>
<body>
</body>
</html>
I’m not familiar with CSS, and I read that you have to declare the language in order for the hyphenation to work.