1. The verb is s’attendre à and not attendre.
   Construction with s’attendre à is :

"s’attendre à ce que* + verb in the subjunctive".

2. You are right, pour cannot be used here.

3. "Exist" is exister in French. We can’t use être in this case.

4. Your sentence expresses regret. French uses the infinitive of the verb (and not a conjugated verb) in interrogative sentences expressing surprise, regret etc. (that will often be the case after comment and pourquoi).

Your sentence could then be :

Comment nous attendre à ce qu’une langue internationale existe ?

But it is not the only possibility as "nous" can be replaced by "se" since this "nous" is a general "nous".

Comment s’attendre à ce qu’une langue internationale existe ?

Your original English sentence had the idea of possibility ("could"). If you want to retain it in French you have to use pouvoir. If using pouvoir "s’attendre" will remain in the infinitive even if used with nous but "pouvoir" can either be conjugated with nous or used in the infinitive.

Comment pouvons-nous nous attendre à ce qu’une langue internationale existe ?

Comment pouvoir nous attendre à ce qu’une langue internationale existe ?

Comment pouvoir s’attendre à ce qu’une langue internationale existe ?

You are making an interesting point in your comment. I only mentioned the most popular form and my answer was incomplete.

Both s’attendre que and s’attendre à ce que exist and are correct. "S’attendre que" is very literary and s’attendre à ce que is a lot more common^*. Personaly I never use s’attendre que.

So indeed you are right you could say :

Je m’attends que tu fasses tes devoirs.

ou

Je m’attends à ce que tu fasses tes devoirs.

About ce que :
Verbs which require "à" before a following noun add "ce que" when followed by a subordinate clause. Here you will find a list of such verbs.

^{*Le TLF about s’attendre que et s’attendre à ce que : L’un et l’autre sont corrects; le 1er est prôné par les puristes, le second s’emploie plus couramment.}

Another possible way to try to add the notion of “could” by using “pouvoir” in the first clause might be:

“Comment peut-on s’attendre à ce qu’une langue internationale existe?”

However, please note that none of the translations for this construction found in the above Reverso Context link actually include could, it being translated instead as “How do we expect …?”

With this “missing notion of could” in mind, perhaps could could be “recaptured/re-emphasized” (hopefully without changing the meaning too much) by using “pouvoir” in the second clause, as follows:

“Comment s’attendre à ce qu’une langue internationale (ne) puisse exister?”

(please note that I am including a non-negative/expletif «NE» in parenthesis because I don’t know if this is one of the cases where it could/should appear)

Finally, to propose a possibly less formal way to capture “How can we expect …?” I’ve seen (again, from Reverso Context) “vouloir” used loosely sometimes to mean “expect,” for example:

“Comment veut-on qu’une langue internationale existe?”

Or to reinstate/re-emphasize the notion of “could” in clause 2:

“Comment veut-on qu’une langue internationale (ne) puisse exister?”