The answer is
seven numbers are given: 1, 3, 2, 6, 7, 5, 4; and the (six) differences between them follow the following pattern: +2 -1 +4 +1 +2 -1; the next logical difference in that cycle (a repeating cycle of differences of +2, -1, +4, -1) is +4, so the next number is 4 + 4 = 8.
As for the languages
1. Icelandic, 2. Portuguese 3. Irish, 4. Spanish (also Portuguese, but since that's "taken up" already by the preceding "três", which cannot be confused with any other language, let's assume it's Spanish), 5. English, 6. French, 7. German or Dutch; I'm choosing Dutch because reasons following below.
Now the pattern in this set that
numbers [1,3] , [2,4], [5,7] are the sets of languages that are most related to each other within the sample of 7 languages given [Icelandic, Irish], [Portuguese, Spanish], [English, Dutch] . I'm specifically associating "English" with "United Kingdom" here, and not with any of the other dozen English-speaking languages in the world. Also note that this means 7. must be Dutch, because both Dutch is linguistically closer to English than German is, and the Netherlands are geographically closer to the U.K. than Germany is.
As for the language of the eight entry, 
it must be in the language most closely related to the sixth entry, , French, to follow the pattern. That's Italian, thus the answer is otto.