chore: add language property to metadata JSON

Author: 3w36zj6

website/metadata.json

@@ -1,5 +1,6 @@
 {
 	"$schema": "./metadata.schema.json",
+	"language": "ja-JP",
 	"version": "0.13.1",
 	"typstOfficialUrl": "https://typst.app",
 	"typstOfficialDocsUrl": "https://typst.app/docs/",

website/metadata.schema.json

@@ -2,6 +2,11 @@
 	"$schema": "http://json-schema.org/draft-07/schema#",
 	"type": "object",
 	"properties": {
+		"language": {
+			"type": "string",
+			"description": "The language of the documentation.",
+			"enum": ["ja-JP", "en-US"]
+		},
 		"version": {
 			"type": "string",
 			"description": "The version of the documentation, without a leading 'v'."
@@ -47,6 +52,7 @@
 		}
 	},
 	"required": [
+		"language",
 		"version",
 		"typstOfficialUrl",
 		"typstOfficialDocsUrl",