I had never traveled outside my country until I went to Disney World last summer
or
I never traveled outside my country until I had gone to Disney World last summer
Until is followed by past simple or past perfect ?
I had never traveled outside my country until I went to Disney World last summer
or
I never traveled outside my country until I had gone to Disney World last summer
Until is followed by past simple or past perfect ?
The pluperfect (or past perfect) would intuitively be further in the past than the perfect tense (as it refers to something already in the past (completed) at a past time.)
The two examples swap the position of past simple and past perfect (as you put it).
The statement that succeeds "until" chronologically occurs after the statement that precedes "until".
Therefore I would be inclined to believe that your first example is correct, and your second example incorrect.
A perfect construction always names an event or state which occurs before a designated 'Reference Time' (RT), the time you are primarily talking about. With a present perfect, the RT is the present; with a past perfect, the RT is some moment in the past. Events and states at RT are referred to with simple or progressive forms.
So what is your 'Reference Time' in this context, which the past perfect 'non-event', never traveling outside your own country, falls before? Clearly it is last summer, when the visit to Disney World occurred.
Since the visit to Disney World occurred at that past RT, you refer to it with a simple past:
I had never traveled outside my country until I went to Disney World last summer.
Note, however, that you are not required to use the past perfect in your main clause. You may use a simple past, traveled, and rely on the until to express the time relation between the earlier non-event and the later event:
I never traveled outside my country until I went to Disney World last summer.
This indeed is the more natural way of expressing the situation you describe, where the visit to Disney World ended the state of never having traveled abroad.
But your second version, with a simple past in the main clause and a past perfect in the until clause, is impossible in the situation you describe. A past perfect must be 'anchored' on a later RT, and this sentence does not provide any RT after your visit to Disney World. It would be valid only if you were American and you were trying to express the thought that you never visited other countries until after your visit to Disney World.
The second version is completely unacceptable, since the "state, condition" of never having traveled abroad existed before OP went to DisneyWorld (so going there came later, not earlier).
With that in mind, we see that the first version validly uses Past Perfect to reflect the "temporal sequence" (whereby never traveled abroad is followed later by went to DisneyWorld).
But note that in practice probably most native speakers wouldn't bother with the more complex Past Perfect form anyway. It doesn't contribute any new or necessary information, since the time sequence is contextually and pragmatically obvious.
In short, although OP's example #1 is "correct", it's not necessarily as "natural" in a relaxed spoken context as Simple Past (I never traveled abroad until I went to DisneyWorld). The guiding principle here is don't use Past Perfect unless you really have to - and if you're not sure, assume you don't need it. The relatively few occasions where you make the wrong choice will be more than offset by the much greater number of occasions where you sound more like a native speaker by not using it.