Japanese is an Altaic language. It is related first to Korean, then Mongol, then Turkic languages. Suffixes are common in all these languages. European languages do not have suffixes like Tanaka-san, Satou-kun or Hiromi-chan. German and Dutch are exceptions. Both have a nearly perfect equivalent for the familiar and affective diminutive -chan. Germans use -chen, as in Gretchen (little Greta). The Dutch/Flemish say -ke or -ken, as in manneken (little boy) or with a given name (Tinne -> Tinneke).

The similarity is striking. It looks the same, and means the same. The Dutch usage is especially widespread. By chance, the Dutch were also the only Europeans allowed to trade and exchange cultural ideas and learning with the Japanese during the Edo period. Could it be that the Japanese copied the -ken and transposed it to -chan ? I know that it cannot be the other way round, because -ken has been use since medieval times in Dutch (well before the first contacts with Japan).

Or would Dutch and German have inherited this from ancient invaders who spoke an Altaic language, like the Huns ? The Huns were after all closely associated with Germanic tribes and merged with them at one point. It could be one of the few words that they left behind them in Europe.
If it is just a coincidence, it really is a good one.