Question about Brennos' interview

[Brennus]

So, would the proper way to use the term Celtic be similar to the proper use of the term "Semitic," to refer to a large group of related languages/peoples (depending on the context) that are more-or-less related to each other? So, no one would speak "Celtic," they would speak a Gallic, Brythonic, Goidilic, Celtiberian, Volcae etc language, which is part of a larger Celtic language family. It seems like splitting hairs, but it emphasizes the locality rather than just painting with a wide brush of "Celtic."

Pretty much. Although you could further split hairs by adding a Belgic linguistic group, assuming you accept the work of Hans Kuhn "Volker schweissen Germanen und Kelten".

What are the linguistic differences though? Modern day England and Germany vary considerably yet both countries speak Germanic languages.

The traditional approach, although an increasing number of phonologists are questioning it, has been to divide the "Celtic" languages (Atlantean has not caught on as a term) into P and Q groups, based on the above differences in translation. It also helps that the extant Q languages all stem from Old Irish whilst the extant P languages all stem from Iron Age British. At present the classification is as such:

Q Celtic:

Irish/Gaelic
Scots Gaelic (Incorrectly referred to by many Scots as "Gallic", also may have some influences from the now extinct Pictish and British Strathclyde languages)
Manx (Extinct as of the 1974, however since revived, also strongly influenced by Early Medieval Norse)

P Celtic

Welsh
Breton (Also influenced by Gaulish)
Cornish (Extinct 1777 as a primary language but undergoing a revival since)

I should also add that all of these languages have been strongly influenced by neighbouring English and French.

Another way to look at it is in terms of it's meaningless within ancient writing. By which I mean, the term Celt (like Gaul) is simply a cover-all term. The nearest modern day example (at least that I feel comfortable using) would be 'Asian'. In certain contexts it can be a pejorative, and it is wide ranging in its use. Imagine some civilisation in a couple of thousand years from now, picking through the remnants of a mostly lost/destroyed literature and trying to piece together the 'Asian' language, orthe original homeland of these 'Asian' people. Perhaps we might have some mythical 'Asian' Kingdom or hegemony developed as a background for this 'peoples'/civilisation.

In short; there are a lot of myths and beliefs around the idea of Celticism and most of these have no real bearing in contemporary historical data or archaeological data.

You haven't read a book by Peter Wells "Beyond Celts, Germans and Scythians" have you? He makes pretty much the same point with regards to the term Asian.