On Google Books, I found a 2010 book on Mood in the Languages of Europe consisting of scholarly articles by various experts. The two German Konjunktiv forms, like the English subjunctives, are examples of irrealis moods. Here is a relevant quotation from the article on Mood in Latvian and Lithuanian:
Requests and expressions of opinion are frequently in the irrealis form for reasons of politeness. […] Yet the suggestion of politeness may be achieved not only by representing the act of volition, opinion etc. as unreal, but also by depicting the object of an act of volition as unreal, or just possible.
Here is another from an article on Mood in Modern Georgian:
The optative [a form of irrealis mood] can be explained in the vein of politeness theory, where indirectness is a form of (negative) politeness: while the imperative is a “direct” expression of volition, the subjunctive does not necessarily require the speaker to be the addresser of an imposing request.
Another from a paper on Mood in Czech and Slovak:
The hypothetical, “non-impositive” semantics of these main clauses is loosely associated with politeness in Czech and Slovak; thus, a 1st person conditional declarative may be used as a polite offer, and a 2nd person conditional interrogative as a polite request.
One from Mood in Swedish:
A further, cross-linguistically well attested use of the preterite subjunctive is to indicate politeness.
Mood in Dutch:
The indicative counterpart of (19) [“this background may serve as introduction”], given in (19') [“this background serves as introduction”], is merely a factive statement and does not suggest any of the ‘hope of approval’ that was expressed by the speaker in the original utterance.
Likewise, the speaker in (17) and (18) would definitely be less polite if he had used an indicative rather than a subjunctive form without applying some alternative politeness strategy.
Mood in Estonian:
The pragmatic use of the condition[al] to express politeness is widespread; it often occurs together with a modal verb […]
I think it’s pretty clear from these quotations what the linguistic position on the phenomenon is: It’s very widespread (though in some languages such as Greek or Russian it doesn’t exist at all or is quite different), and the use of irrealis moods for politeness tends to be related quite logically to other typical uses.