For the record: Gaelic refers to the Celtic languages spoken in Britain, not Gaul. The term you are looking for is Gallic.

The team has created Celtic and Germanic voicemods based on reconstructions of the languages. Especially the Germanic one is highly speculative. It is assumed there were several "branches" of the Celtic language in EB's time already: the three Celtic factions of EB1 use Gallic (P-Celtic), but a few Briton units were supposed to get a Q-Celtic voicemod. I am not sure if they ever got round to doing that. Incidentally, the Celts of southern Gaul did have writing systems: they used Greek signs and IIRC an alphabet of their own as well.

As a note of caution: the Armenian alphabet may date back to 5th century AD, but you cannot assume that pronunciation hasn't in changed since then. Unless it has been completely cut off from the outside world, a language will always be changing. And, obviously, before a language is "centralized" (by having an alphabet and uniform spelling) it will vary a lot over time and space.