Re: Submerged Languages

The standard term is "substratum" or "substrate". A substrate language is what you are describing as a "submerged language".

A substratum language is spoken of when a population has undergone language replacement. In ancient France, Romance replaced the Celtic language of the Gauls, but left certain remains, both in  vocabulary, and in phonology. In other circumstances, even a few grammatical elements are passed on to the successor language.

A substratum is to be distinguished from a superstratum and an adstratum (or, superstrate, adstrate).

A superstratum is what one language imposes on another in terms of vocabulary, phonology, and even some grammar,  but not to the point where language replacement occurs. English has a huge French superstratum dating from 1066 an all that.

Adstratum and superstratum seem to be used interchangeably, but as I understand it, an adstratum is what you call the strong influence of one language on another at a particular time in history. In 1100, French was an adstratum language to English; nowadays, it's a (historic) substratum.

Mark.