Asterix would have spoken Gaulish, a Celtic language that we have a few recorded instances of: http://www.orbilat.com/Encyclopaedia/G/Gaulish_language.html

Gaulish, Proto-Irish, and Proto-Brythonic would have been relatively similar, but Irish and Brythonic went in very different directions. By the time Old Irish and Old Welsh developed, neither would have been mutually intelligible with Gaulish at all. Irish maintained the old declension systems while the Brythonic languages dropped it. Irish also evolved a system of slender and broad vowels that cause palatalization of nearby consonants. Modern Irish, Scottish Gaelic and Manx Gaelic (the Goidelic branch of the Celtic languages) both branched off from Irish, while Welsh, Cornish, and Breton descend from Proto-Brythonic. All of these languages make use of verbal nouns, a feature not seen in Gaulish. None of them are conservative enough to bare much resemblance to Gaulish and Latin is, at least superficially, much more similar.

Gaulish was, according to contemporary sources, very similar to Latin. It's very likely that Latin (an Italic language) and the Celtic languages shared a common ancestor, Italo-Celtic. Italo-Celtic would have broken off from Proto-Indo-European, before splitting into the Italic and Celtic branches, though not all linguists agree.

I would recommend asking over at /r/Linguistics. They might have a better insight.