Switch locale to C

Remove the explicit installation of an English language pack and instead
just default LANG to C. This settls Perl's noisy warnings and also stops
any side effects of different language build hosts.

Note that this is set right at the start of the build process, so it
should be entirely possible to override if needs be.

Change-Id: Id3b31162d4198fa02dc5a4d11168e57dbcd14a5d