Hello 
Thanks for checking it. 
Well, in Hungarian we have plural but not always which makes it little difficult. But let me explane some simple examples.
Most of Hungarian words have plural version.
If it’s uncountable.
E.g.
Group → Groups = Csoport → Csoportok
Topic → Topics = Téma → Témák
So it think that external library is correct in that we have plural form.
But we don’t use plural if there is a number (count) before words.
If it’s countable.
E.g.
1 group → 2 groups = 1 csoport → 2 csoport
1 topic → 2 topics = 1 téma → 2 téma
If you check the Hungarian language file you can see the plurals are always the same.
Hovewer I see there are strings which is not use count on form one.
Like this:
It’s ok because we don’t need the count in this case when only 1 topic it’s pretty obvious. But the other one difference here is only that the other is appears a number. The text is same no matter there is number or not.
So if you ask me I just would remove the one form.
But if it’s coming from an external library which is I think correct, because Hungarian have plural form but not most of cases that Discourse uses. So probably adding the one form to have the abality to customize it would be the easiest solution to prevent the issues. Yeah it is mooostly same but at least we can customize it in admin.
I don’t know it’s a hard question. As everyone who running a Hungarian forum and made customization in admin have been changed only the other form. Because it fell back to other from one always. So if we add the one form, admins have to review again all of the plural strings to modify that version too to be the same as other.
Thanks 