In his Origenes Antwerpianae, sive Cimmeriorum Becceselana novem libros complexa of 1569, Johannes Goropius Becanus argued that Dutch (or perhaps Flemish since Antwerp is in Belgium) is the original language. Like Sir William Jones' famous paragraph that kicked off comparative-historical linguistics, this fact can been gleaned from most books that deal in some way with the historiography of linguistics. And the belief that one's native tongue just happens to be the oldest and most original of the world's languages is still alive and here. Three examples will suffice: Andis Kaulins' case for the Latvian origin of Indo-European, Nostratic, et al., Keerthi Kumar's Dravidian origin of Indo-European languages, and Matej Bor's Slovenian origin of Venetic. I've also seen a book that connect Chinese and Sumerian. What brought all this up? Well, I'd been wallowing in the whole Shakespeare authorship question recently, and that just about leads to anything.
In re Goropius, read Professor Reinier Salverda's inaugural address delivered at University College London in 1990.
Here's the Jones paragraph for completeness' sake: "The Sanskcrit language, whatever be its antiquity, is of wonderful structure; more perfect than the Greek, more copious than the Latin, and more exquisitely refined than either, yet bearing to both of them a stronger affinity, both in the roots of verbs and in the forms of grammar, than could possibly have been produced by accident; so strong indeed, that no philologer could examine them all three, without believing them to have sprung from some common source which, perhaps, no longer exists; there is a similar reason, though not quite so forcible, for supposing that both the Gothick and the Celtick, though blended with a very different idiom, had the same origin with the Sanskcrit; and the old Persian might be added to the same family..."
[Jones. Collected Works, vol. III, pp. 34-5]