As a Finnish speaker, I just can't see generic localisation systems like Fluent working with agglutinative languages. For instance, consider the Polish example for Firefox Account from Fluent's front page:
-sync-brand-name = {$case ->
*[nominative] Konto Firefox
[genitive] Konta Firefox
[accusative] Kontem Firefox
}
In Finnish, this would be called Firefox-tili. Tili means account and belongs to the Kotus type 5 words, which specifies how the word is inflected with various suffixes. Fluent doesn't support agglutination, so you'd have to specify every form of the word separately:
-sync-brand-name = {$case ->
[nominative] Firefox-tili
[partitive] Firefox-tiliä
... 10ish more inflections
[nominative first person] Firefox-tilini
[partitive first person] Firefox-tiliäni
... 10ish more inflections
... the above inflections but for 2nd and 3rd persons
[nominative first person plural] Firefox-tilimme
[partitive first person plural] Firefox-tiliämme
... 10ish more inflections
... the above inflections but for 2nd and 3rd persons plural
[nominative first person questioning] Firefox-tilinikö
[no idea what this is even called] Firefox-tilittömänäkin
[no idea what this is even called 2] Firefox-tililleensäkään
... lots more
}
Ideally, you'd only specify that Firefox-tili is a type 5 word and the system generates all of that boilerplate.