Created attachment 80995 [details]
Tatar language was present in the language list in Preferences - Languages section, but I cannot see it anymore.
This is rather important because it is impossible to do spell-checking on Tatar, and highly inconvenient.
Thank you for your bug report, I can reproduce this bug running LibreOffice Version: 18.104.22.168.beta2
Build ID: 33224f4f11a05cfad2249e812fcc2975fbb61f6 on Mac osx 10.8.4.
Comment on attachment 80995 [details]
The screenshot shows the list of default document languages. Tatar was never present there as we don't have a locale data set for Tatar [tt-RU] and fully supported default languages need locale data assigned. You'll find Tatar in the language list of character attribution though, for example in Writer menu Format->Character, which makes it available for spell-checking if a corresponding dictionary is installed.
For how to submit locale data please see https://wiki.documentfoundation.org/LibreOffice_Localization_Guide/How_To_Submit_New_Locale_Data
Meanwhile tt-RU locale data was added in 2014 ...