Does it handle the "x of y" in Slavic languages correctly? For example in Polish: "Page 3 of 4" is "Strona 3 z 4" "Page 3 of 100" is "Strona 3 ze 100"
This is an excellent question and a very good use-case. I'm a Polish speaker myself, so I can definitely relate. It's also a good excuse for me to talk a little bit more about the advanced features of Fluent. The Fluent Syntax is a simple declarative DSL. By design, it doesn't allow translators to build complex conditionals or use arithmetic. There is, however, an escape hatch. The problem you described can be solved…
Right now I'm using qt translation system, it handles nicely various plural forms, just like Fluent.
But it requires special code in each message that has "X of Y" to handle the "ze 100" correctly in Polish. It might be done for Polish, because we have Polish developers, but many languages have similar quirks and it's not done for them. And it would result in combinatorial explosion of translation message versions if source code had to add special case for each quirk in each language.
This seems to be a much better solution.