I would continue to use whatever layout I was using before up until the point where my speed or convenience starts to suffer.
Once I find myself trying to type stuff that'd be easier with a native layout, I'd change. Not a moment before.
In my case, chances are I'd be still writing code mostly, for which my layout is perfectly adequate. The odd native-language text is no big deal, not worth switching to a different layout just because of that. Only when those become the majority.
Granted, when trying to use someone else's keyboard, it becomes a bit of a problem. But that's why you tell people to make it easy to switch between their layout, and yours (and in turn, you set up your computer to be able to switch to their layout aswell).
At work, I have three layouts: the plain old boring US qwerty, hungarian qwertz (for those collegues who prefer that), and programmer dvorak. Takes a single key combo to switch to any of them, and everyone's happy.