AMX Mod is multilingual since version 0.9.9: the server can be configured to display messages in other languages than English, and clients can choose their language as well.
A. Server Language - Console
By default, AMX will be in English on the server. This is set in addons/amx/config/language.ini, but you shouldn't have to modify this file.
If you want to change the server language, use the amx_language command on the server console:
This is what happens if you haven't added any languages yet.
If you add some languages and don't make AMX support them 100%, you will be displayed something like:
Now you can change your server language by appending the language code to the amx_language command:
If you select a language that is not fully supported, you will get an extra line:
AMX will then fallback to English if a message is not available in the translations for this language.
Now that you've changed your server's language, you might want to save it:
This way, your setting will be kept even after a map change or a server restart.
B. Server Language - Menu
Any user with the ADMIN_RCON (l flag) access level can modify the server's language with an in-game menu. The menu is called with the amx_langmenu command, and can be used to choose the server's and the client's (see below) languages.
As you can see here, option 4 is used to set the server language (selecting it several times will cycle in the available languages), and option 5 saves the setting (just like amx_language save).
C. Player Language - Console
Players can also use the amx_language command:
Players can choose two languages: a primary language, and, if this one is not fully supported, a secondary ('fallback') language. Both can be set with the amx_language command:
If a player selects non-fully supported languages both as primary and secondary, AMX will fallback to English if a message is unavailable in these two languages.
Players can also save their settings:
This just sets their setinfo _lang to their primary (and optionally secondary) language(s).
D. Player Language - Menu
Any user can call the language menu thanks to the amx_langmenu command. Of course, non-admin users can't change the server's language.
Option 1 cycles through the available languages. Option 2 is not available unless the player selects a language in 1 that is not fully supported.
At last, option 3 saves the configuration (issuing a setinfo _lang on the player's console, just like amx_language save does).