Making a promise, taking an oath; such activities uphold the integrity of the speaker and thus the very act of swearing - jurer for the French - is closely linked with notions of sanctity. Hardly surprising, then, that Europeans can be seen invoking the name of God left, right, and centre whenever they make a promise. In the same way that Obama proclaimed I swear to God ! on the day of his inauguration, Spaniards can be heard extolling Te lo juro por Dios !, or rather Ich schwore bei Gott if you're German and Te lo giuro su Dio! if you're Italian. Another good way to seal a promise is to put a price on your mother's head ( I swear on my mum’s life), or Te lo giuro su mia mamma in Italian. Only the Polish prefer to put their love for their grandmother on the line, literally claiming to swear as much as I love my grandmother (Jak babcię kocham).
Anything goes insofar as it is necessary to convince one's interlocutor of one's good faith. And if they are neither God nor grandmother-fearing, then there are still many well-loved idioms to fall back on. The French, unafraid of placing all their eggs in one basket, do not hesitate to combine all possible proofs of their good faith: Promis, juré, craché proclaim adults, whilst children prefer to go to hell if they tell a lie Croix de bois, croix de fer, si je mens je vais en enfer; thus echoing the English lines Cross my heart and hope to die, stick a needle in my eye. The Germans, on the other hand, make their promises by invoking the bones of their ancestors, as was the custom in the 16th century: Stein und bein schworen.
The Spanish, weary of such age-old formulas, instead choose to turn them on their heads, mockingly proclaiming to swear by Snoopy (Te lo juro por Snuppy), . To call on the witness of this harmless cartoon dog leaves little hope that the promise thus made will ever be realised. You can also count on the Germans‘ Indian promise (Indianerehrenwort, Scout’s honour).
The Polish, however, are keen on mixing up their pastries: Jak bułkę kocham, rogalik świadkiem (I swear as much as I love cake, the loaf of bread can bear witness). But never fear, if you have no intention of keeping your promis then all you have to do is cross your fingers whilst making it. That works no matter where you are in Europe!