Мне следует перевести какие-то документы каких-то других клиентов.
...Because I know very well documents which documents I will translated I use то instead of нибудь.
Exactly. You kinda got the principle. The grammar is ok!
But in Russian if we don't want to specify something to someone we say "кое-каких".
This implies I know what exactly but it's not your business. (Absolutely neutral, doesn't imply any offence)
also we have кое-кто, кое-что, кое-как, кое-где, кое-когда и т.п.
Your sentence means that you don't know for sure now which documents from which clients you have to translate.
But these documents and these clients are already specific, tangible, you may learn what they are somehow.
Maybe you can learn it tomorrow, when you come to the office. But now you only know that you have some documents from an unknown client to translate. Maybe a friend called you and said that you have an order, but did not specify the exact details.

So:

I don't know but I have an opportunity to reveal it (e.g. if I ask him what he is doing) (specific, tangible) - какой-то
Look, he is translating a document. Посмотри, он переводит какой-то документ.
He is already translating, the document is defined (by the history) although unspecified.

I don't know and I have no chance to reveal it (any, vague) - какой-нибудь
When you'll be translating a document don't forget about this rule.
Когда будешь переводить какой-нибудь документ, не забудь про это правило.
The document is undefined and unspecified because it hasn't happened yet.

I know but not telling - кое-какой
Don't distract him, he's translating a document.
Не отвлекай его. Он переводит кое-какой документ.
The document is defined but unspecified in a way that it's kept a secret even though I know what the document is.