The message that is displayed when kicked for spamming is not translated, instead appearing as disconnect.spam. This has been an issue since the creation of that message in Minecraft 1.3.
Connect to a server, and start spamming the chat. After roughly 10-12 messages, you will be disconnected for spam. (You must not be OP on the server, as OPs are exempt from the spam check, although I've found that spamming a lot as an op and then deoping myself can trigger the spam check in a way that more consistently creates this behavior).
You should receive a disconnection screen saying disconnect.spam. If it says instead "Disconnected" (disconnect.genericReason) or "Disconnected by Server" (disconnect.disconnected), try again.
disconnect takes a String which is used to create a text component, not a translation component. While there is a translation value defined for disconnect.spam (Kicked for spamming), it goes unused because the text is not translated.
- Send a translation component instead of a text component, so that disconnect.spam is actually translated (like the other disconnect translation keys)
- Pre-translate that message (as with other messages sent in that context) to the current English version ("Kicked for spamming")