It is usual in European countries to use the comma as a decimal marker, and the full stop as a ‘thousands’ separator. This does not apply to the British Isles, where the same system as in the USA applies.

Well for kroner yes. A full stop is used as the thousand separator, yes. The comma is also used the opposite way, e.g. NOK 29.999,99. Sometimes a space is used to prevent confusion especially when referring to foreign currencies, e.g. $29 999.

There are dialects, too. Some are heavy to understand. The majority language, Norwegian, has two distinct written varieties: Bokmål (‘Book Language’) and Nynorsk (‘New Norwegian’). They are so close to each other linguistically that they may be regarded as «written dialects», mutually completely intelligible. Orally, local dialects are extensively used throughout the country and will differ from the written language. They are also mutually intelligible, although they can be very diverse, not least in intonation. In addition, both Swedish and Danish are closely related to Norwegian and thus intelligible to Norwegians, although with some initial difficulty in many cases.
In speech, there is much blending between dialects, Bokmål and Nynorsk, but in writing, the varieties are kept more clearly apart. Nynorsk is mostly used in Western Norway as a written language (by roughly 10 % of the people, amounting to about half a million), Bokmål is dominant in the rest of the country, and is used in writing by close to 90 %.

All languages change over time. Old Norse is considerably different.