Currently the formal and informal variants are a bit hit and miss. The fact that not every message needs translation (compare with variants of English) makes them problematic. For languages which want to take seriously, we could make the formality an inline feature in addition the existing PLURAL, GENDER and GRAMMAR features.
Possible approach:
The formality could be an additional option in the user preferences (like gender) or driven by the language codes directly. Language could choose their own number of formalities, not limited to two.
Example (current solution):
(es) ¿Estás {{GENDER:$1|seguro|segura}}?
(es-formal) ¿Está {{GENDER:$1|seguro|segura}}?Example (proposed solution):
(es) ¿{{#FORMAL:Estás|Está}} {{GENDER:$1|seguro|segura}}?The underlying assumption here is that only MediaWiki uses these formal/informal variants. If other projects use them, they could keep them as separate languages still until a better solution for them is described.
(Originally posted on https://www.mediawiki.org/wiki/Language_goals_and_wishlist#Better_support_for_formal_and_informal_variants)