The most well known foreign language university in Japan is Sophia University/Jochi Daigaku located in Tokyo's Chiyoda Ward. They have classes both in English and Japanese and accept undergraduates.
They are expensive however and your estimated cost just for *Freshman* year alone according to them will be the following (in Yen):
Entrance Fee 270,000 (once at matriculation)
Tuition Fee (assuming 31 credits: 31X30,100) 933,100
Building & Maintenance Fee 180,000
Laboratory Fee 19,500
Correspondance Fee (Including Consumption Taxes) 5,300
Orientation Fee 17,600 (once at matriculation)
Alumni Association Fee 5,000
Health Insurance Fee 2,500
Personal Accident Insurance for Students Pursuing Education
and Research 2,550 (for a 4-year period)
Total fees for freshmen 1,435,550.00 JPY = 16,129.78 USD
This doesn't even cover your living costs and lodging.
In addition you will need a financial/general guarantor (typically a Japanese resident sponsor) before you will even be considered for enrollment.
This is true for any college, university or vocational school you are considering in Japan.
Here's their website:
Sophia University English Site| Sophia University Official Website
ICU or Internation Christian University would be the other well known alternative and has both English and Japanese classes but they too are also expensive and the same rules apply.
Here's their website:
ICU International Christian University
Unless money is no object and you can shoulder the expenses involved you may want to reconsider and possibly finish your education in Portugal first and then travel to Japan on vacation/business travel to check things out then you can see about post-graduate courses or perhaps exchange programs.