Drop the legacy language alias into target_language on each set
BEFORE schema validation (the pre-v1.2 alias rule, see
docs/concepts.md), so a legacy manifest validates instead of tripping
the strict additionalProperties / missing-target_language rules.
Drop the legacy
languagealias intotarget_languageon each set BEFORE schema validation (the pre-v1.2 alias rule, seedocs/concepts.md), so a legacy manifest validates instead of tripping the strictadditionalProperties/ missing-target_languagerules.