I thought szivesen would be said as ' you're welcome ' after someone thanks you.
Possibly; of course I cannot speak for all English speakers, but welcome is not a usual response to thank you in any English I have ever heard. I think it is more likely a mistranslation.
You're welcome isn't as popular anymore, but instead things like: no problem, no worries, cheers - not omitting the you're
Yes, it definitely looks like a mistranslation, especially with the exclamation mark at the end. But I have heard it without "you are". Maybe the speaker had actually said (or thought) it, only the sound waves never made it outside their head.
The correct translation for "Welcome!" would be
"Isten hozott (téged/titeket)"
"Isten hozta (Önt/Önöket)"
The words in parentheses are usually not added. This greeting literally means "God brought you".
And, interestingly, it is usually used with the locative:
"Isten hozott BudapestEN", and not "BudapestRE".
I definitely believe you! With maybe 2 billion speakers, that some say welcome in response to thank you is not a surprise.
There is no equivalent of the Académie française to determine Standard English, but the problem is bigger than that. Since it is so usual in Europe and India (as examples) to use English as a "shared" language, among people who don't speak the same native language...who is to say their usage is "bad"? If it is consistent and understood by everyone?
Yes, absolutely, the most widely used language is also the most abused. Other languages must protect themselves (Hungarian is also ruled by an Academy, and there are most probably many others), English simply absorbs everything.