Install `lang` for en_US installations

Currently pmboostrap installs lang (and musl-locales, but this can be dropped since it's now a dependency of lang) if the requested language is not en_US.UTF-8.

https://gitlab.postmarketos.org/postmarketOS/pmbootstrap/-/blob/472726a9dcb6dd10369d9c16fa55519320d83219/pmb/install/_install.py?page=2#L1282 (in pmbootstrap@196186df)

This makes sense unless we expect the user to be able to change the current language.

For example, in g-c-c the langauge selection is empty since no locales for other languages are installed. Especially with the rise of immutable filesystems and image-based installations, we can't expect the user to manually install the lang package to change the UI language.

I think lang should be part of _pmb_recommends of postmarketos-base-ui. And the referenced part of the pmbootstrap should be removed.