Communicating with summoned spirits

There are multiple ways of summoning spirits in the game, and it is often mentioned that one of the more common methods of getting a spirit to do something for you is to bargain with it.
To bargain with a spirit one needs to be able to communicate with it somehow, and it seems more than a little far-fetched to assume all spirits happen to be able to speak in some language you know, but the books always seem to assume there is no actual problem to start bargaining with some summoned spirit.

So how does one communicate with a summoned spirit in general, so one can bargain with it?
I am thinking in particular of Airy Spirits, but the question covers spirits in general.

Develop an InMe spell for it?

A possible solution if you are a Hermetic magus, but there are many other people in Mythic Europe who can summon and bargain with spirits.

For example, take a look at the canon NPC "Nicolai, the Summoner" from RoP:tI p119. How does he bargain with the spirits he summon? He can't cast any spells. From his description it seems he mostly summons Airy Spirits who seem unlikely to know any human languages.

Well most infernal spirits do have the ability to speak all earthly languages. It's really more ghosts or spirits from an ancient culture which tend to lack this virtue, and the obvious answer is to learn the related language.

From RoP:M p100:

For want of a better term, a spirit's communication with physical beings is called Silent Speech, for such impulses are usually received as audi-tory species, but it could be interpreted as smells or feelings on the skin, and still be un-derstood with no difficulty.

Which seems like it is saying communication with a spirit is no problem as long as you can perceive them.

True enough as far as it goes, but there are plenty of other sorts of spirits - elemental spirits being some of the more common, and they seem very unlikely to have any knowledge of human languages.

It does seem to say just that, yes.
That would seem to answer my question - communication with spirits is typically no problem.

Yje magic realm chapter on spirits certainly makes it seem as though spirits do not require a language ability- even the ghosts do not have those abilities (do ghosts forget how to read since they don't have language abilities?) On the other hand what is stated is that the soundless emissions from spirits can be understood regardless of how they are perceived, but the other question is whether spirits can inherently understand the mundane species emitted by mortals...

According to Legends of Hermes (pg 47), Fortunata of Jerbiton "translated dead languages by commanding ancient ghosts to read to texts aloud while she used Mentem magic to understand their words", so I don't think it's automatically true that you understand anything a spirit is saying (you could argue about whether that's specifically an issue of the fact that they're reading the text, but it still implies spirits can speak in specific languages). The various spirits in that chapter also vary as to the language abilities that they have - but the ones that don't have them are usually further away from being "people" (i.e. they're the storm spirit and the Imagines).

Eh, sort of. Even for that we know

Those who can understand the silent speech of spirits (such as those with Second Sight) may “overhear” the basic silent speech

So it's not automatic for everyone. But I do suspect a large number of summoners have Second Sight or similar.

Yeah, that is because you probably need to be able to percieve the spirit in some way in order to be able to understand its Silent Speech. Second Sight is one obvious way of doing that, but far from the only one.