Well, generally it's not a problem to add a new language, but regarding Gaelic, I am not 100% sure.
Every localization of avast! program is done via a translation tool (Passolo) and it doesn't "know" such language (even older versions of Windows SDK don't know the language code). I guess I can define a custom, unnamed language code in the project - but I am not completely sure if other problems won't appear later. Hopefully not, but...
I should also warn you that it's a lot of work to translate avast! UI - almost 30 000 words in English.