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.