In-House or Freelance Copywriter
I would say the advantages of using a freelance, external copywriter is that you will get a different perspective, experience level and maybe a fresher approach than having your own in-house one.
Also, a freelancer is easier to change of you don't like them.
An in-house copywriter will give more stability on some ways. You'll get to know what they can do, they will fit in with your in-house style more easily and you only go through the hiring process once.
Availability is also a factor. Can you hire someone locally who is a fluent native speaker, in the language you're producing copy in?