Wh-clause after a preposition

Solution 1:

In my experience, it is speak to, not speak for, and yes, it can be used with an interrogative content clause:

The constant clashes between locals and immigrants speak to how difficult it is to incorporate foreigners into our society.

Your examples with speak for sound like an error to me, but if there's a dialect where either example is accepted, then I would expect both examples to be accepted (since I'd expect that that dialect simply uses speak for in all cases where I use this sense of speak to). I do find a (very) few examples online, such as "all of this just speaks for how difficult it is for plastic to bio-degrade" and "I feel that that speaks for how difficult it is to be at the top level in ski racing".

Solution 2:

Adding to @ruakh's great answer, a complement of a preposition (also known as object of a preposition) is, most of the time, a noun or noun phrase. Therefore, as long as the wh-clause is a noun phrase, it can be a complement of a preposition.

Wh-words such as what, why, which, where, when and how are all used in forming an interrogative sentence or interrogative content clause which can be used as a subject, object and subject complement as in:

What's missing here is authenticity. (Subject)

I don't know how you did it. (Object)

That's how it is. (Subject complement)

"How difficult is it to incorporate foreigners into our society?" is a question and if you change the word order from is it to it is, it becomes an interrogative content clause which can be a complement of a preposition.

A side note: It sounds more idiomatic if you insert well between speak and for as in speak well for.