What is the plain alternative to “fossilized language”?

Solution 1:

The use of the adjective fossilised meaning:

set in a rigidly conventional pattern of behavior, habits, or beliefs; "obsolete fossilized ways"; "an ossified bureaucratic system" fossilized, ossified.

refers to terms and expressions which were used back in time, probably some decades ago, but that are still effective from a political point of view to communicate with that specific electorate. A use of a more contemporary language would not produce the same response with those people.

Old-fashioned language may be a more common alternative.

Solution 2:

outdated language

outdated

no longer in use or fashionable; out-of-date; outmoded; antiquated.

Solution 3:

When used in conjunction with language, the word fossilized has a special meaning.

A “fossilized form” as used by linguists means an idiom, word, or even morpheme that has become frozen in time, and so is no longer otherwise meaningful or productive in what should be corresponding situations. It can happen to individual words or to entire phrases.

A few of the scores and perhaps hundreds of possible examples include: Be that as it may, God save the King, to and fro, short shrift, kith and kin, getting one’s just deserts, run amok, flotsam and jetsam, hither and yon.

In the broader context of your quote, the fossilized language referred to may well be the empty language of formulaic — and therefore false — courtesy, like the honorable gentleman (who is unlikely held to be either) and by your leave (because I’ll just go ahead and do it anyway).

Solution 4:

Stilted

(of behavior, speech, or writing) too formal and not smooth or natural: