They can be from anywhere, but it requires you to have answered the call to arms. It's not just a chance for citizens to fight for their home, the Lothian Empire, but also a chance for mercenaries or soldiers of fortune. So if you're not a citizen of the Empire, then you're likely a mercenary, either from the empire, another one of the other nations, or even overseas in say Dyfed or Themyscira.

Your character can also be one of the soldiers stationed at Garleton, be they Lothian or Reynes.

Naturally though there are limitations, such as being a Lyonesse soldier is not possible... At this point in the RP at least.

As the RP goes on, there will be chances to create characters of different backstories. For example, once the story reaches Lyonesse territory, you can make a character who is a Lyonesse soldier, that has chosen to defect.

So to answer. Yes. They can be from anywhere, within reason.