The visa thing sounds like a back-formation unique to your mother tongue. The word "visa" is the past participle of videre (Latin for "to see"). So it is not a plural in its formation/derivation.
You're mistaken. The verb is vidēre, videō, vidī, visum.
There is also the noun vīsum, -ī, plural visa with the primary meaning the seen [thing], appearance, perception. This is the word that was borrowed into English and other languages.
Why not look it up before you assert someone is wrong? It's explained in detail in [0]. It's clear that visa is used as a past participle, not as a noun, and it's applied in the singular. There is no support anywhere for the use of visum as a singular and visa as a plural. Which is what I believe I said above.
