zompist.com: 'Though Syldavia is in the Balkans, Syldavian forms are usually closer to Dutch than to German (cf. güdd, nietz, wertzragh above). Extratextually the reason is simple: when he needed foreign words, Hergé regularly used Marols, the Brussels Flemish dialect his grandmother spoke. (Another example is the Arabian city of Wadesdah— "What is that" in Marols; and see the Arumbaya in L'oreille cassée.)
'Some readers conclude that Syldavian "is" Marols, but this is an exaggeration...'