Besides, I prefer to keep things as 'historically realistic' as possible. Latin is a dead language, therefore making it, but that is just my humble opinion, unfit as a steppingstone for any future language in the first place, excépt as a source for loanwords and neologisms.
Well, Hebrew was a 'dead' language until in in the late 20th century Ivrit was developed as a continuation of biblical and mishnaic Hebrew. Also, how big a language community is might have little influence on whether it will be spoken on colonies, due to a founder effect. I think, though, that French just fits the Gallentean picture far better than ancient Greek.
Good point. However, I do think that the revival of old languages will not occur randomly with every possible speech. That this Hebrew intiative was succesful has partly to do with profound religious reasons, along with of course social ones. It is not unconceivable that things like this will happen in the distant future again with e.g. (Classic) Arab. Religion as a life-support system for dead languages aren't uncommon: Sanscrit, Summerian, Hebrew, Arab, Latin... This principle is particularly interesting for Amar(rad?)
On the other hand, languages are revived for reasons of cultural identity, like Cornish. Only time will tell whether this will be succesful, but I think succes at least for some part depends on: 1) having a territory, like a state or province with language borders, 2) having media, 3) having education and from there textual production. These infrastructures provide a stable ground for further develpment, and when lacking, I do wonder whether learning the language is still worth while.
The consequences of the lack of such a ground can be seen all around the world: Evenki in Russia, Ainu in Japan, Bask in Spain/France, you name it. Even Flemish Dutch in Belgium, although there the required 'infrastructure' has been built in the course of the 20th century (I keep it simple). Anyhow, as a further elaboration on this point: it is estimated that world wide there are about 6000 languages, but most of them are minorities and dying, and certainly will die in the next two centuries. At the same time we see globalization and the spread of liberal capitalistic democracy (or at least the first part) and a movement towards a mono-culture. In this, and this is nothing but a prediction, doesn't allow all those 6000 languages to exist. Think of the substrate effect described earlier. Even if they still exist, they will undergo changes. Speakers will probably 'absord' in the more prestigious language community (unless there are strong nationalistic and religious reasons). Reasoning forward from the mono-culture, the chance that colonies are founded by English, French and Chinese speaking people (which is not the same as actual English, French and Chinese) is quite large. The development of the European Union could allow German to play a large part in it as well.
But colonies aren't founded for no reason. Even if there were some Greek native speakers who founded a colony, then there is still a huge chance that lots of non-Greek migrate to it because of the advantages the colony has to offer (be it economy, living space...). And for reasons mentioned, 1000 Greeks cannot overcome 10.000 English or Chinese, especially not in a global-interstellar society in which English and Chinese are already dominant (because the migrants are thus part of a larger akin community, with schools, political entities etc.). It reminds me of the Afrikaners in Johannesburg after Transvaal became economically interesting: the influx of migrants was so big, that Afrikaans more or less disappeard from the scene, making Johannesburg an anglophone city.
Another reason for believing in just a handful of dominant languages is deliberate assimilation. Johannesburg turned Anglophone, therefore providing the English rulers in Cape Town with an excuse to declare war on the Boer republics and 'help' their fellow anglophones. In the '30's, the Germans occupied Austria, Bohemia and wester Poland partly because these regions were linguistically and culturally related.
Of course this doesn't rule out the possibility of a substrate effect the other way around: minorities may alter the dominant language on a colony too. E.g. Afrikaans has a slight influence on South-African English.
Anyhow, these are some ideas/theories of mine.
As possible substrates for some primordial form of Gallente we could think of German and English. We could also assume a scenario in which many Spanish speakers migrate to the early colonies from South-America for economical reasons, thus creating a Spanish substrate.