Lets put all the dictionary related bugs in one place to reduce duplication and as it has its own git repo. https://cgit.freedesktop.org/libreoffice/dictionaries/ Marco is maintaining a github project to keep the english dictionaries updated[1] and also maintains a tool[2] for editing the Dictionary, Thesaurus, Hyphenation, and Autocorrect files. [1] https://github.com/marcoagpinto/aoo-mozilla-en-dict [2] http://marcoagpinto.cidadevirtual.pt/proofingtoolgui.html
TDF Wiki also has a page about dictionaries. https://wiki.documentfoundation.org/Development/Dictionaries
Proofing Tool GUI webpage is now: https://proofingtoolgui.org