Comment 10 for bug 1576282

It's both. LOCPATH needs to be set when launching the application because otherwise an absolute path will be used. And locales-all contains data such as time and date formats or how to deal with numbers.

I created a proof of concept snap providing the same languages available on the click-based phone images. That brings the 127M of installed size down to 34.5M - the snap (as noted in https://github.com/ubuntu/snapcraft-desktop-helpers/pull/12) is indeed a mere 3.5M, which is nice for OTAs, but it grows when unpacked. Just to highlight why there's different numbers being talked about.
https://code.launchpad.net/~ubuntu-sdk-team/+snap/ubuntu-language-pack-base