Rule: Which in direct and indirect questions has definite reference; what has indefinite reference.
Examples of errors:
(1) and (2) contain embedded or indirect questions equivalent to the following direct questions:
What language is spoken by the parents?
What new directions could be given to their lives?
Which is possible in these sentences, as well as in (3), but only if the head noun has definite reference, that is, if it is one of a definite subset of the class of entities represented by the noun. This condition does not obtain in (l)-(3), but compare:
The mother speaks English and the father Spanish. I don't know which language (i.e. of the two) the child speaks.
He can become a criminal or a good citizen. He will have to determine which direction his life will take by himself.
The housewife could be paid annually, monthly, weekly, or daily. Which sort of payment would be most appropriate?
Which implies an of phrase (which need not be expressed) that specifies the subset of the class of things referred to; it asks for identification of one or more members of this subset. What is used when no subset has been identified. When the subset is identified immediately afterward, which or what may be used (which in this case has cataphoric reference):
What/which film would You like to see--Urban Cowboy or Lili Marleen?
What and which function as determiners in (l)-(3), but exactly the same difference between them obtains when they are used as pronouns; what is an indefinite and which a definite pronoun, e.g.:
"What do you want to do?"
"Well, we can watch TV or go to the movies."
"Which would you rather do?"
In informal speech, as in this imagined conversation, what might occur instead of which, even though the reference (to either watching TV or going to the movies) is definite. Here is another example:
We have chocolate, vanilla and strawberry. Which/what flavor do you want?
In this case the lack of strict cohesion between sentences that is characteristic of informal speech accounts for the possibility that the definite reference (to the three flavors) may simply fall to be carried over in the mind of the speaker from the first sentence to the second.