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.