The active sentences here have an unexpressed subject for the verb phrases to answer and to translate - at least semantically. They mean something along the lines of:
- That is an interesting question (for someone) to answer.
- That's an easy sentence (for someone) to translate.
In some theories of grammar, they would actually stipulate that there is an actual non-overt pronoun in the space where the brackets are in the examples above. It has the flavour of a non-definite third person pronoun. Note that if we actually stipulate specific subjects for these verbs we are obliged to use the subordinator for:
- That's an interesting question for Bob to answer.
- That's an easy sentence for her to translate.
This omission of subjects in constructions using to-infinitives is quite common. Compare the following pairs of sentences:
I want him to leave.
I want it to continue.
- I want __ to leave.
- He wants __ to continue.
This invites the question who do you want to leave? Or Who do you want to continue? The subject of the verb phrases to leave, to continue seems to be the same as the subject of the verb want. The last two sentences could be illustrated thus:
- I want (myself) to leave.
- He wants (himself) to continue.
These examples seem to show that infinitival clauses without overt subjects are still interpreted as semantically having subjects.
The passive sentences referred to in the question are a bit ambiguous to me. I'm not sure if I'm actually able to read them as the same type of sentence as the others. The to here seems to have a modal flavour. They seem to be the same type of sentence as:
- That is a question to be answered.
- That is a sentence to be translated.
The use of the infinitive here seems to imply that someone ought to answer the question or translate the sentence. It is perhaps this ambiguity which prevents the general use of the first constructions in the passive.