You only have to pay if you're buying Discord Nitro, which, from what I know of it, is basically a way to support Discord that gives you a few perks (such as using emoticons from one server in any other) as incentive to buy it. However, if you're just interested in using Discord without Nitro, it costs nothing.
Discord describes itself as an "all-in-one voice and text chat for gamers". You can communicate with others by direct-messaging or "calling". You can also create/join a "Group DM", which is the equivalent of a group chat, though you can only invite your Discord "friends".
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When you create a Discord account, you receive a four-digit tag. (For example, Username #9385.) Other people that have Discord can use this tag, along with your username, to find your account and message you. I suggest not giving around your username/tag to just anyone, and instead saving it for friends/people you have arranged to meet. You never know what some people might do.
As for your last question: I myself do not know of any built-in translator on Discord. However, you
should be able to make it work if you look into the options.
Now, if you'd like to research more aspects of Discord that I myself cannot provide, I suggest visiting the official site.
https://discordapp.com/
