Hey Zeke, I'm answering you but just using VW's post to do it.
Well we get the 'idea' that he can't understand them from the scriptures you quoted there. It isn't just some arbitrary point of view about tongues pulled out of thin air. 'No man understands him' (of course it can be argued that is 'man' and not the devil) and 'howbeit in the spirit he speaks mysteries' as you quoted. Well, a mystery is a mystery. And I can reasonably deduct that it is a mystery to all but God. As a matter of fact, if you study 'Vine's Expository Dictionary of New Testament Words', and I am quoting this from memory, so cut me some slack if it isn't exact, but he says the word 'mystery' is 'that which is out of the range of unassisted natural apprehension and can be made known only by divine intervention', or in other words by revelation of the Holy Spirit. He assists us in our apprehension of the mystery. I don't think the Holy Spirit is assisting the devil in comprehension of what is being spoken.
Of course then it could be argued that the devil(s) doesn't need assistance because he can understand it (from Zeke's point of view). But let's look at what the purpose of tongues bypassing our understanding would accomplish. One thing it accomplishes since it bypasses our understanding and intellect is that it bypasses our unbelief. If we don't know what we're praying, then we can't get overwhelmed by the mystery of what we're praying, and add unbelief to it because it's beyond our faith level at the moment. That's on our level. On the devil's level, it would be so that he doesn't know the plans of God in our behalf and come up with counter measures in order to thwart what God is doing in answer to "who cares if he understands your prayers".
To further the concept, we are ambassadors for Christ. I won't do it here but when you study the whole concept of what an ambassador for a country is, then certain details as 'secret communication' between the ambassador and his home country come to light. What can be more destructive to the devil than God showing us the devil's plan which He knows, and praying the perfect prayers beyond our own understanding and the devil's to thwart those plans.
Anyway, there's more there, but you get the idea. I think there is more to support that the devil doesn't understand from what the scripture says in 1 Cor. 14 than it does that he does understand. Especially when one realized we are in 'warfare' with an enemy and then you begin to liken that to natural warfare, where one just doesn't reveal their plans to the enemy. God is the supreme military strategist.