How could the people from Nirn name the Akaviri swords Katanas, when there were no portuguese explorers to transmit the word? I know the Tsaesci are supposed to be based in Japanese culture, but that's a little unbelievable.

From what I understand is that the Tsaesci unsuccessfully invaded Tamriel. From that encounter the swords became known to Tamriel. As for why they are called katana, probably for the same reason Altmer are called high elf and Dunmer were called dark elf. That is, we the Nirnless need a familiar name to be able to assimilate with an alien culture. Easily corrected if you can create a mod to rename all katana to something more Akaviri sounding.