I found a workaround:
I went to the folder /opt/onlyoffice/desktopeditors/dictionaries and modified an existing language that I don’t use.
Since I use pt_BR for my daily work, I repurposed the pt_PT language to use a custom dictionary I made for the Xavante language, using the .dic and .aff files from GitHub - edurbs/libreoffice_xav: Custom xavante spell checker for LibreOffice.
Now, whenever I want to spellcheck a document in Xavante, I just switch the spellcheck language to pt_PT (Portuguese - Portugal), and OnlyOffice will use my custom dictionary for Xavante.