As the title says, language dictionaries installed from this page: https://addons.thunderbird.net/en-uS/seamonkey/language-tools/ can be installed in SeaMonkey without any problem, but cannot be used.

After installation, when going to the Spellcheck pane in the Preferences window, it says that there are no dictionaries installed and only the Download more dictionaries (translated on the fly from Italian, string text might not be 100% accurate) menu option is available. The installed dictionary cannot be selected and it never comes into play in the program.

I noticed that dictionaries are now webextensions also for Seamonkey - is it possible that this is the problem?

I forgot to mention that this has been verified by three different people (all with the Italian dictionary).

dictionaries in web extensions format are currently not supported. We will fix this in a future release.

Use the latest non web ext version from the dictionaries version history entry and disable updates for this one for the time being.
For Italian should be:
https://addons.thunderbird.net/en-US/seamonkey/addon/dizionario-italiano/versions/5.0.0

I encountered the bug also for installing German and Croatian dictionaries.

Frank's workaround works, but it takes some time to find how to get at the old dictionaries (in Spell checker Click Download more dicionaries..., Add-on page for specific dictionary opens, do not install dictionary on top of page but scroll down to Version Information, open the drop-down list, Click See complete version history, choose first dictionary without "webext" in its suffix, then click on Add to Seamonkey, restart Seamonkey)

@Frank-Rainer: Going to the webext format is an enhancement, but that the enhancement does not work is a bug and needs a fix, as you said. Can the bug and the workaround (the route to the old dictionaries) and that it will be fixed be filed at the known bugs at the Release Notes?

Thanks!

This has been fixed for SeaMonkey 2.53.16b1. Just forgot to change the release notes.