I’m not sure languages:preferred is a great key name for this suggestion. The term “preferred” to me implies a hierarchy: “OSM is my preferred geo database; I prefer OSM to Google Maps”.
This feels problematic particularly applied to minority languages. A minority language might be second in sign order (e.g., bilingual signs in Kashubian) but tagging that the majority language is “preferred” seems ripe for misunderstandings and conflict.
Conversely, two languages might have equal legal status, and they are ordered purely for visual and practical reasons (e.g., the New Brunswick example of space-saving signs printed “Boul Babineau Blvd”), where tagging languages:preferred=fr;en also doesn’t seem great since it’s not actually preferred.
If the purpose is to order languages purely for visual rendering and the intent is to mimic local signage, I would suggest something like languages:order_used_on_signs or languages:sign_order or something else similarly descriptive that does not imply a hierarchy. Unfortunately it’s likely to be a longer key name.