Some languages have multiple local variants, e.g. en-US, en-UK, en-ZA etc. In fact, they may have dozens more which don't have any specific definitions. And if such a locale is used in LO, we don't get to use various mechanisms which are currently locale-specific with no language-level fallback. One of these is the Thesaurus.
A recent example has been en-IL: bug 156101. That one was solved by manually adding it as one of the locales covered by en-US.
I suggest that, as a general measure, there will be a fallback to a language-level default thesaurus/dictionary file/fileset, so that the same procedure need not be repeated whenever one uses an uncommon locale we have not specifically catered to.
As I've mentioned in bug 156101 comment 14, I think this should be covered by a solution for 83561.
To me, it doesn't make sense to have a language variant's fallback mechanism that is functional for spellcheck, but not for e.g. thesaurus. I don't think we need that complexity.
In my opinion, this should be closed as a duplicate of bug 83561 (just like bug 154366).
Otherwise, make those two blocked by bug 83561.