In the original French it was: "Nous parlons pour Mitan...euh...nous martons pour Pilan".
The first bit translates as "We're speaking for Mitan" but I cannot figure out the second bit.
Thanks for providing the original French version, mct16, which is always interesting to compare. From a quick bit of online translating, "nous martons pour" translates as "we hammer for" which is as deliberately meaningless in the context of the scene as their previous mangled attempt, "we speak for". Obviously, what they're attempting to say is "Nous partons pour Milan."(We leave for Milan.)
It's neat the way Hergé is able to be so precise with the spoonerism variations, swapping the middle consonants of the verb and the place name in the first mangled attempt, and swapping the first letters in the second attempt. It helps that both parlons and martons are actual verbs in French, although Hergé does have to use non-existent place names (I think!) with Pilan and Mitan. Michael Turner's version is less precise with the letter swaps but keeps all of their attempts within actual words. It'd be interesting to see if in his new English translation for the e-book version, Michael Farr followed, in this instance, Michael Turner's tendency to prioritise natural sounding and properly funny English, or if he went for a more direct translation of Hergé's spoonerism constructions, ie: "We're lealing for Mivan ... erm ... we're meaving for Lilan."
I don't have the e-book versions, but I'm guessing Michael Farr will have aimed for something funnier than that!