I don’t think, based on the way keyboard assignment is done, that it would work on a default-per-app basis. Currently, that is.
In this link (which for some strange reason redirects itself up to the main directory instead of the sub-directory I intended), if you look in the data/terminal sub-directory, there is already a fr.yaml terminal-style file, as well as the us.yaml. I think all one has to do is to copy the particular language layout, copy the terminal extra keys, merge them into one, and then save as whateverlanguage.yaml in .local/share/squeekboard/keyboards.