There's a great isolate of Manchu being spoken in Xinjiang, who are descendents of some kind of military outpost that was abandoned after a dynastic change back in China proper.

Also, you can find some speaking of Astur-Leonese and Aragonese languages on Spain :

https://en.wikipedia.org/wiki/Astur-Leonese_languages https://en.wikipedia.org/wiki/Aragonese_language

This languages are romanced derivative and could you think as brother/cousins of Spanish

Can you give us some more information?

I know that it is not an authoritative source, but there was an episode of an Anthony Bourdain show which travelled to Mexico / MidWest kind of area where they spoke Spanish that was mostly unchanged since 400 years ago.