You always register a trademark for a geographical region (smallest is one country, biggest is "the world"). The bigger this "region" is, the more it will cost you (just an example).
If you decide not to register in a certain country, someone else can do it with the same name and force you to remove your app in this countrie's app store. So will have to decide which countries are your primary market and then register the trademark there.
BTW: You will also have to choose between "word" or "logo" trademark and the registration costs multiply for each category you want it to cover (there are about 40-50).