Taxation.
Taxation issues between countries are treated via Taxation Treaties. Most countries have treaties between themselves. This is to avoid double taxation.
The only exceptions are countries where there is no income tax, i.e U.A.E., Saudi Arabia, ect...
You can google most of these taxation treaties using the names of the countries involved. Most of the treaties are copy and paste of each other. Look for the Section titled Dependent Income. It is usually Clause 15 or 17 of the treaties.
Hope this help.