The Franks, strangely enough, spoke Frankish. There isn't a lot of Frankish spoken today, it evolved and merged into modern German. The appended language tree showws he place of Frankish in the Western German derived languages.
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It was Charles the Great who made the Frankish kingdom an
Empire.
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W.M Frankish has written:
'The wonder book of why and what?'
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The Franks spoke Old Frankish, which eventually evolved into Old Dutch, Middle Dutch, and then Modern Dutch.