"Itu adalah setelan-setelanku."
Terjemahan:They are my suits.
"that is" is singular. "they are" and "those are" are plural. If translation was from 'itu adalah setelanku' then "that is my suit" would be correct as setelan might be singular. As Indonesian sentence includes; setelan-setelan which is plural you cannot translate it to "that is".
Please note that 'that' is a complex word and can be used in many ways (a bit like you might use 'yang' in many different ways) so would not always be translated as itu when it is used in some contexts.
In a similar way this=ini (singular) and these or they=ini (plural)