This was brought up in https://ask.libreoffice.org/en/question/78708/i-have-downloaded-the-dutch-thesaurus-the_nloxt-in-lo-522-but-it-doesnt-work-it-installs-but-doesnt-show-in-writer/ I downloaded the latest version of the Dutch thesaurus from http://www.opentaal.org/projecten/synoniemen.html and took the 3.x version. Installs fine in LO and AOO 4.1.2, runs in AOO (after some hassles) but refuses to work in LO 5.2.2. The thesaurus item in the Tools menu stays grayed out, the Ctrl+F7 shortcut doesn't work either. But the extension works fine in Apache OO 4.1.2, so the extension isn't to blame. Btw: I have the 5.2.2 full version, not the rc, but the release version isn't listed yet.
Confirmed with 5.2.2.2 / Windows 7. Works in 3.5.0.3. => regression At the same time, the builtin English thesaurus works, so I wonder what the difference is between the two.
The Dictionaries.xcu contains malformed property key for supported locales, so it doesn't work. (To fix it manually, extract the said file from the OXT archive, replace all 4 <value>nl_XX</value> tags with a single one, separating their values with spaces, like "nl_NL nl_BE .....", then re-add the corrected file back.
Worked like a charm, thanks, Urmas.
Thanks for the hint, Urmas. Closing this as NOTOURBUG, then. Peter, could you get in touch with OpenTaal, and ask them to fix their extension? Also, they're welcome to submit it to LibreOffice, so it's shipped with a Dutch thesaurus, and maybe check if the Dutch dictionary is up-to-date.
I sent a message to their mailing list. Thanks for the input. :)