How to install extra dictionaries?
During a Windows installation, you have the ability to choose a custom install. By doing this, you are able to choose both the different menu languages from a large list, and choose various dictionaries to be included in that installation. If you do not choose a custom installation, LibreOffice will install a set of dictionaries by default.
After you install LibreOffice on your system, whether it is Windows, macOS, or Linux, you have the option to add more dictionaries for your use.
There are a large number of .oxt dictionary, thesaurus, and other language aid add-ons for LibreOffice. There are two good places to find these .oxt dictionary spell checking files. One is at the official Extension site for LibreOffice, and the other is at the North American Community DVD Project's dictionary page(s).
The official Extension Center is at the following URL:
Choose the option of "Dictionaries of different languages for all program modules" with the "Any version" secondary search option chosen. This will list all of the .oxt files that is listed within the Extension Center pages.
Once you find the dictionary, or dictionaries, you wish to add to your installation of LibreOffice, they you can install these .oxt files through the LibreOffice Extension Manager. To get to this Extension Manager, click on the Tools menu option and then the Extension Manager option in the bottom section of that menu option list. At this point, you click on the "Add button" and browse through your computer's drive system. At that point, you click on the .oxt extension file [be it a dictionary, template, gallery, or other .oxt file] and click on the "Open button". Then you follow the automated steps to complete the installation of the .oxt extension.
Also, within the Extension Manager, you have the ability to enable and disable and non-locked extension list there. If you decide to stop using an installed dictionary, you can disable it instead of removing it from your LibreOffice installation. This makes it easier to switch between different dictionaries of the same language, but a different variant. There are several languages that have multiple possible dictionaries to use. There are 20 Spanish dictionaries localized by country. You can then install one for Spain, Mexico, Peru, Panama, and Venezuela, and choose which version of Spanish to use while working on your Spanish language document. All you have to do is disable all of the different Spanish dictionaries except the one you want to use for that document.