What does "1.234" mean?
To an American or a Brit, it's one and a bit. To a German, a Spaniard or an Italian, it's one thousand two hundred and thirty-four. Same characters, two numbers a thousand times apart.
Now put that in a quantity field on a checkout page, or a price in an import file, and think about who reads it which way.
Numbers feel like the one part of an app that doesn't need localising. A number is a number. But the way people write numbers, the units they measure in, and the way they write phone numbers all change from country to country, and all three end up in your forms every day.