or if we can register as one international company?
No, sorry. Same like you can not have a "multi city postal code". THere is no such thing as an international company you register at - you always register in a specific country.
Depends a lot what you do. In general sales offices are not assumed to be businesses butt dependant locations. You mostly need to register locally to pay... people (employment taxes, social security) or they work as independants (run their own business) and invoice you. YOu may also want a local bank account to run your expenses account and have some local place for people to pay.
Whether you move further is your decision - depends legally also on country - and the next step is a wholly owned daughter company... which is a company the "mother" owns 100% but it has local management etc... and that is EXPENSIVE (not to register, but multi company accounting, local management all cost).
What would be the smartest, cheapest, easiest thing to do?
Partner with a local business or have self employed sales agents working for you - this means you do in both cases not deal with any intricacies and keep the overhead small. Once a particular country expands, you ahve already made provisions allowing you to set up a local branch.