You use the player index when your only sending the client the message. You use LANG_PLAYER When you do print to all client. You would do the same thing with show_hudmessage.
Code:
client_print(0, print_chat, "%L", LANG_PLAYER, "TEST");
If you want to send a message using the server language. You would replace id or LANG_PLAYER with LANG_SERVER.
Edit:
P.S. LANG_PLAYER was design to show the message in the client language vs just one language
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