language selection in pmbootstrap

Languages are different in musl-libc based Alpine than in your typical glibc based distro:

• locales are not generated via locale-gen. instead, there is a musl-locales package, which makes these locales available. Just install the package, no need to build them afterwards.
• set a language with e.g. export LANG=de_DE.UTF-8. There is no localectl or similar (as provided by systemd). Instead, we could put that line in an /etc/profile.d script.
• install lang to pull in all language subpackages of installed packages (install_if)

With that in mind, we could provide language selection in pmbootstrap as follows:

• ask in "pmbootstrap init" about the language of the to-be-installed postmarketOS
  • there aren't many languages now, so we can just hardcode them in pmb/config/__init__.py
• if it is not english:
  • install "lang"
  • write /etc/profile script to select the language