Yeah this is where these things fall apart. I haven't used Mozilla's Fluent but I used a very similar closed source system at another company some years back. Some failure modes:

- Gender agreement is not trivial. In French, in "Mary bought it" the verb needs to match the gender and number of the object not the subject: "Marie l'a acheté" vs "Marie l'a achetée" vs "Marie les a achetés" depending on the gender/number of the "it" object. But in most other cases the verb needs to match the subject in gender and number, in Polish "Maria kupila" vs "Stas kupil" vs "Oni kupili".

- In many languages nouns need to agree in case, gender, and number with the phrase they're in, even in English we see this with pronouns: "this is he" vs "this is his".

- And not to mention number agreement between pronoun and either subject or object, depending on context: "this is the button" vs "these are the buttons" - but also "this hovers over the button" vs "these hover over the button" etc. Pronouns in general are a world of hurt, as are copulae (is/are/etc)

- So once we want more complex sentences, simple word tagging like $gender becomes insufficient, because now there's multi-party agreement to worry about, we have to worry about $gender_subject and $object_gender_number_case, etc.

This becomes completely untenable for all but the most technical translators. Maybe those are easy to find for a world famous project like Firefox. Unfortunately, not so for a run of the mill commercial project.

It's certainly complicated but I think there is no way around it if you want to have correct translations. The same kind of system is used for translating MediaWiki and it seems to work great there. Example message: https://translatewiki.net/w/i.php?title=MediaWiki:Logentry-b...