For some reason many languages use an irregular system for numbers. Well, reasons are historical.. things may have made more sense in (e.g.) Old Norse than modern Danish, for example.
Japanese though.. now that's regular. One, two, three, .. nine, ten, ten-one, ten-two,.. ten-eight, ten-nine, two-ten, two-ten-one.. and so on. And then 'hundred', and continue the same way. to-hundred-to-ten-five (225). The only stumble is when you pass 10000, as from then on the grouping is in tens of thousands, not in thousands. But with that the only issue there's only one struggle to master (for learners coming from a culture which groups in thousands).
Anecdotally, Japanese children learn to do arithmetic quicker than children from (e.g.) Sweden. But I've been unable to find real scientific confirmation of this.
