"Banks do not open on Saturdays."
Translation:Bank tidak buka pada hari Sabtu.
Literally 'Bank tidak buka pada hari Sabtu" means "the bank does not open on Saturday" (THE bank, singular)
The Indonesian sentence does not refer to a specific bank, a singular bank.
It refers to banks in general.
If it refers to a specific bank, THE bank, then a determiner would have been used.
Like this :
Bank itu tidak buka pada hari Sabtu.
As given the phrase is ambiguous regarding plurality
It's not ambiguos regarding plurality.
The Indonesian sentence can only refer to banks in general, not a specific bank.
Surely you need "bank-bank tidak membuka pada sabtu" to make it match the English, yes?
No need to reduplicate the noun when talking about it in general terms.
Other examples :
Saya suka apel = I like apples (apples in general, not just this one specific apple)
Harimau adalah hewan = Tigers are animals (all tigers, not just this specific one)