Set a language for a scanner sitting #58

Outcome. A collector sets one language for a scanner sitting, and the printings offered and added are in that language.

• Language is part of the catalog entry, not of the lot (ADR 0019), so this chooses among printings.
• Only useful where the instance's catalog holds that language.
• Reverses spec 009's deferral of sitting-wide defaults (ruling 20).

A candidate from the roadmap. It is refined into a spec when its turn comes, and checked then against the north star's hosted-door rules and spec rules (one outcome, one PR; a spec that adds collector-owned data extends the export in the same PR).