"The porter comes out from the hotel in which foreign tourists are sleeping."
Translation:A portás abból a szállodából jön ki, amelyikben külföldi turisták alszanak.
Külföldi is foreign as originating from a foreign country.
Idegen is stranger, someone or something one doesn't know. However, sometimes in English you say foreign when in Hungarian idegen is fitting.
idegen nyelv = foreign language
"Overseas" is a non-thing in Hungarian, you shouldn't try to reconstruct it. (It's meaning is silly in English too, anyway. In Britain, Germany is not overseas, but China is. I'm yet to meet a person who could justify that.)
Of course, there is a word, "tengerentúl", which perfectly describes "overseas". Overseas countries can be "tengerentúli országok",
overseas shipping can be "tengerentúli szállítás",
Of course, it is up to our location to determine what we call "overseas". Maybe it can be overseas because of the way of transportation, even if there is a way to go inland.
On "külföldi", it literally means "belonging to outside land". That is, from another country.
"Idegen" can be anything foreign, alien, unknown, not belonging somewhere.
This question has come up several times in this course. How I understand it is that in this type of construction, where you have "abból ..., amelyikben..." abbol can be translated as "the." This is because the section starting with "amelyikben" (I forget the grammatical name for it) identifies the hotel. It's the hotel in which foreign tourists are sleeping. So, when translating to English, you don't need "that hotel" because the word "that" also identifies which hotel is being referred to, which would be redundant.
That said, I don't think all of the exercises in this course are consistent about this. Initially, most of the sentences had literal translations to English. Over the months since the beta version was released, they have been fixing these, but it's not 100% completed. So you'll see a sentence now and then in which Duo insists that you should say "that ..., in which..." This is frustrating, especially when you're not sure which way it's going to go, as you're entering your translation. The best advice I can offer is: Report, report, report. And then be patient. :)
Editing for a final comment: When you translate this type of sentence from English to Hungarian you do need to keep "that". Even if the English sentence says "... the ...., in which ..." you need "az a" or "abbol a" or whatever.