For example, a Pokémon Go account can be logged in in Germany, and then I want to log it in Australia, does it work? Are there any region limit?
Pokemon GO makes no attempts to limit where you can play the game -- after all, that would be counterproductive to the point of the game. Even more so with regional exclusives.
However, cheating the game through use of GPS spoofers and other methods may result in your account getting soft-banned or terminated. Typically, though, this check is pretty lenient -- if you suddenly teleport to Japan, the game will lock up. However, flying to Japan will not trigger the softban in most cases.
As far as I'm informed there are no real regional limits, but you might get a "Soft Ban" if the app detects that you are traveling faster than possible.
- if you're logged in in Germany, log out, and 5 minutes later log in at a position that isn't possible to reach in that time e.g. the United States.
- If you're logged in in a car or train or something that drives faster than a person could drive with a bicycle.