No, I think that that is OK and expected behavior The names of the dictionary files are vec.dic and vec.aff as you can see and that is what you see in the Language sub menu.

I don't think that your response is the right one. I, for example, installed the Catalan dictionary (https://addons.mozilla.org/en-US/firefox/addon/general-catalan-dictionary/) and it displays Catalan even if its files are ca.aff and ca.dic. However, surprisingly, your answer worked: I renamed my files into Venetan.aff and Venetan.dic and it displays "venetan". Anyway I would like it to be internationalized. However, I wonder why the Catalan works and why, renaming my files with a capital V, I then see the language with a small v...

From https://developer.mozilla.org/en-US/docs/Mozilla/Creating_a_spell_check_dictionary_add-on : "The .dic and .aff files must be placed in a subfolder named dictionaries within the ZIP file. Both files must have the locale code as their file name." May be because Firefox lacks my language?

I filed a bug to Bugzilla@Mozilla for that.