If you also need the index from the mount journal I would generate a table in-game with [spellID] = journalIndex, because it appears to be an alphabetically sorted list rather than a static value and I wouldn't trust it to be the same across localizations.
Something like this..
Lua Code:
local SpellToJournal = {} -- [spellID] = journalIndex
for i = 1, C_MountJournal.GetNumMounts() do
local _, spellID = C_MountJournal.GetMountInfo(i)
SpellToJournal[spellID] = i
end
Edit: Nimhfree confirmed that the indexes are definitely not the same in different languages.