刀って英語でなんて言うの？

The word "katana" is very commonly known around the world so if you were to use it, most people would understand you. If not, the phrase "Japanese samurai sword" should help your listener to understand exactly what type of sword you are talking about. I hope that this helps. :)

"Katana" is the Japanese word that translates in English to "Japanese sword", this sword was usually used by the Japanese samurai.

The Katana is often referred to as a 'Samurai Sword' in western culture, and is depicted in many movies as such.
The Katana is a traditional Japanese weapon used before the introduction of firearms and after also. Tourists try to take these weapons as souvenirs or gifts to their native country, but because of the lethality of these swords many countries have banned the importation of them.

Forge' means 'to fashion' or 'to make into.' In English, we refer to the bottom of the handle of a sword as the 'hilt.'

Alot of people would actually know what a 'Katana' but if not then you would explain it's a 'samurai sword' you could also call it a 'Japanese sword' or a 'Japanese samurai sword'
tradiitonal means it has been around for a long time or is part of a countries tradition

A Japanese KATANA is known as 'a samurai sword'
'The man who was arrested was carrying a samurai sword.'

This weapon is thought of as very iconic of Japan and of Japanese Samurai culture. It is depicted as such in many films and TV shows.