Making it plural doesn't always mean "replace one word by another".

The right thing to do it:

add_one = "Add one thing" add_multiple = "Add {n} things"

Then you'll provide the full sentence for each language. Of course some languages will need more cases, like slavic language where it's 1, 2-4, 5+, so depending on the languages you need to support you need to put more than 2 strings.