build_image: install lang and musl-locales as part of every image

Localization and internationalization are important topics, specially for those people that are not devs used to work with English and C locale by default. Since bpo-installed images are mostly aimed at less technical users that will not use pmbootstrap, installing those packages here would greatly simplify the process that those users need to follow to get the system in their language. It would no longer be necessary to open a terminal and install some packages.

I guess this has the problem of increased size for images. I understand this probably won't get merged in a single go, and there might be some objections. But I want to get the ball rolling about improving the translations and localization issue, since today I've already fixed half of the issues I had in my backlog about this :D