Use SITE_LANGUAGE environment variable instead of LANGUAGE

[wesleybl]

The LANGUAGE variable is already set and used by the OS.

Fixes plone/volto#6404

[davisagli]

This is technically a breaking change, but I think we should do it since the current environment variable conflicts with the one from Linux.

Once we have a release that includes this and update Volto to use it, we need to update the Makefile and docs in volto.

[stevepiercy]

Once we have a release that includes this and update Volto to use it, we need to update the Makefile and docs in volto.

Can y'all create issues and either link to this PR or create sub-issues in plone/volto#6404?

[wesleybl]

@stevepiercy I created plone/volto#6747

[stevepiercy]

@wesleybl that issue captures the docs todo, but what of the Makefile todo?

[wesleybl]

@wesleybl that issue captures the docs todo, but what of the Makefile todo?

@stevepiercy I've updated the issue to address both.