The process of adding languages in LibreOffice is documented here:

• [https://wiki.documentfoundation.org/LibreOffice_Localization_Guide/How_To_Submit_New_Locale_Data]
• The Locale Generator (for generating new locale data)
• How to add the new language

Save the resulting xml file in the techdoc/proof/spelling/hfst/locale folder.

Thereafter, the new language must be added to LO. Register at [https://bugs.freedesktop.org/], and file a bug saying the language is missing, with the xml file as an attachment.