Include input & option descriptions as data in registry (#533)

* Actually include input & option descriptions as data in registry, not just comments

* Use CLDR as currency code reference

* Add xml:lang attributes to <registry>, <input> and <option>

* Wrap all descriptions in <description> elements

Author: eemeli

spec/registry.dtd

@@ -1,11 +1,17 @@
 <!ELEMENT registry (function|validationRule)*>
+<!ATTLIST registry
+    xml:lang       NMTOKEN  #IMPLIED
+>
 
-<!ELEMENT function (description,(formatSignature|matchSignature|alias)+)>
+<!ELEMENT function (description+,(formatSignature|matchSignature|alias)+)>
 <!ATTLIST function
     name           NMTOKEN  #REQUIRED
 >
 
 <!ELEMENT description (#PCDATA)>
+<!ATTLIST description
+    xml:lang       NMTOKEN  #IMPLIED
+>
 
 <!ELEMENT validationRule EMPTY>
 <!ATTLIST validationRule
@@ -20,14 +26,14 @@
 
 <!ELEMENT matchSignature (input?,option*,match*,override*)>
 
-<!ELEMENT input EMPTY>
+<!ELEMENT input (description*)>
 <!ATTLIST input
     values         NMTOKENS      #IMPLIED
     validationRule IDREF         #IMPLIED
     readonly       (true|false)  "false"
 >
 
-<!ELEMENT option EMPTY>
+<!ELEMENT option (description*)>
 <!ATTLIST option
     name           NMTOKEN       #REQUIRED
     values         NMTOKENS      #IMPLIED

spec/registry.md

@@ -32,7 +32,7 @@ The registry contains descriptions of function signatures.
 
 The main building block of the registry is the `<function>` element.
 It represents an implementation of a custom function available to translation at runtime.
-A function defines a human-readable _description_ of its behavior
+A function defines a human-readable `<description>` of its behavior
 and one or more machine-readable _signatures_ of how to call it.
 Named `<validationRule>` elements can optionally define regex validation rules for
 literals, option values, and variant keys.
@@ -74,6 +74,11 @@ which provide shorthands for commonly used option baskets.
 An _alias name_ may be used equivalently to a _function name_ in messages.
 Its `<setOption>` values are always set, and may not be overridden in message annotations.
 
+If a `<function>`, `<input>` or `<option>` includes multiple `<description>` elements,
+each SHOULD have a different `xml:lang` attribute value.
+This allows for the descriptions of these elements to be themselves localized
+according to the preferred locale of the message authors and editors.
+
 ## Example
 
 The following `registry.xml` is an example of a registry file
@@ -84,7 +89,7 @@ For the sake of brevity, only `locales="en"` is considered.
 <?xml version="1.0" encoding="UTF-8" ?>
 <!DOCTYPE registry SYSTEM "./registry.dtd">
 
-<registry>
+<registry xml:lang="en">
     <function name="platform">
         <description>Match the current OS.</description>
         <matchSignature>
@@ -165,7 +170,7 @@ A localization engineer can then extend the registry by defining the following `
 <?xml version="1.0" encoding="UTF-8" ?>
 <!DOCTYPE registry SYSTEM "./registry.dtd">
 
-<registry>
+<registry xml:lang="en">
     <function name="noun">
         <description>Handle the grammar of a noun.</description>
         <formatSignature>