add huld docs

docs/customization/localization-huld.md

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+# Localization / Translations (HULD)
+
+Hercules servers are able to provide some sort of multi-language thanks to a feature 
+called Hercules Ultimate Localization Design (HULD for short).
+
+
+## How it works
+
+Translations are stored in PO files, a standard format for translations. Those files
+are stored in Hercules and follow a structure that imitates the NPC folder (plus
+a few other files).
+
+During startup, Hercules loads those files as additional languages and whenever
+a NPC message is displayed, the player's current language is used.
+
+A default language is set in the configs, and additional languages may be selected
+by each player by using the `@lang` command.
+
+
+## Generating translation files
+
+In order to start translating the NPCs and other translatable messages, you must
+first generate the translation files.
+
+To do that:
+
+1. Build your server, including HPM Hook plugins
+2. Build the `generate-translations.c` plugin (see [HPM](./hercules-plugin-manager.md))
+3. Run `./map-server --load-plugin HPMHooking --load-plugin generate-translations --generate-translations` this will:
+   1. Start map-server with "HPM Hooking" and "generate-translations" plugins loaded
+   2. Ask map-server to actually generate the translation files
+4. Once it complete, map-server will automatically shut down, this is expected.
+5. You can now see a folder called `generated_translations` in your repository root
+6. This folder contains everything you need to translate, see [Adding a language](#adding-a-language)
+
+
+## Adding a language
+
+Once you have the translation files folder (either downloaded from somewhere or generated with [Generating translations](#generating-translation-files)),
+you need to add the folder path to `db/translations.conf`.
+
+For example, if you have the `Spanish` language files at `db/Spanish/`, your
+`db/translations.conf` should look like this:
+
+```
+translations: (
+	"db/Spanish",
+)
+```
+
+From now on, `@lang Spanish` will give a player the Spanish translation.
+
+
+## Default language
+
+The server's default language is defined in `conf/map/map-server.conf`:
+
+```
+	// When employing more than one language (see db/translations.conf),
+	// this setting is used as a fallback
+	default_language: "English"
+```
+
+Following the example above, you could change the value here to `Spanish` for players
+to start their characters having the Spanish translation. (They could switch later on)
+
+
+## Translatable strings
+
+By default, Hercules will pick every string used in `mes`, `mesf` and `select` commands,
+but you can explicitily tell Hercules that strings in other parts of your script
+must also be translated by using the `_()` macro.
+
+For example:
+
+```HercScript
+.@status1$ = "Closed"; // This won't be translated
+.@status2$ = _("Available"); // This will be translated
+```
+
+
+## Caveats
+
+### Global messages
+
+Messages sent to many players at once, like broadcasts will use the attached player
+language.
+
+This is for performance reasons, since sending messages in each player language
+would require building separate packets/messages for each player (or group of players).
+
+
+### "Unattached" messages
+
+When a message is sent by a script that doesn't have a player attached,
+the message will be sent in the server's default language.
+
+This is for performance reasons, since sending messages in the default language
+would require building separate packets/messages for each player (or group of players).
+Additionally, since no player is attached, there is no player language to use.
+
+```HercScript
+prontera,150,150,0	script	test huld	4_F_SISTER,{
+	mes("Text with player attached. In player language");
+	announce(_("Text in player language for everyone."), bc_all);
+	initnpctimer();
+	close();
+
+OnTimer1500:
+	// Player is not attached here.
+	npctalk("Hi, i'm talking in server default language.");
+	end;
+}
+
+```
+
+
+### Player gender
+
+There is no way to have variation by player gender for the same message.
+
+One strategy to work around that is to include a space at the end of the message
+for one of the genders, for example:
+
+```HercScript
+if (Sex == SEX_MALE) {
+	mes("Hello");
+} else {
+	mes("Hello ");
+}
+
+```
+
+This would generate 2 messages for translation.