Add hi, pl, ko to JSON Schema language maps

The schema was added before the Hindi (73b0c68) and Polish/Korean
(118e4bd) instructions landed, so languageMap and languageStepsMap
still only declared en, es, it, tr, ru, zh.

All 1,324 records carry all nine languages and the README documents
nine, but `additionalProperties` let hi/pl/ko through unvalidated and
unrequired — a record silently missing them still passed validation.

Declare all nine in `properties` and `required` for both maps.
Verified: the 1,324 records validate cleanly against the updated
schema, and a record missing hi/pl/ko is now rejected.

data/exercises.schema.json

@@ -17,9 +17,12 @@
         "it": { "type": "string" },
         "tr": { "type": "string" },
         "ru": { "type": "string" },
-        "zh": { "type": "string" }
+        "zh": { "type": "string" },
+        "hi": { "type": "string" },
+        "pl": { "type": "string" },
+        "ko": { "type": "string" }
       },
-      "required": ["en", "es", "it", "tr", "ru", "zh"],
+      "required": ["en", "es", "it", "tr", "ru", "zh", "hi", "pl", "ko"],
       "additionalProperties": { "type": "string" }
     },
     "languageStepsMap": {
@@ -31,9 +34,12 @@
         "it": { "$ref": "#/$defs/steps" },
         "tr": { "$ref": "#/$defs/steps" },
         "ru": { "$ref": "#/$defs/steps" },
-        "zh": { "$ref": "#/$defs/steps" }
+        "zh": { "$ref": "#/$defs/steps" },
+        "hi": { "$ref": "#/$defs/steps" },
+        "pl": { "$ref": "#/$defs/steps" },
+        "ko": { "$ref": "#/$defs/steps" }
       },
-      "required": ["en", "es", "it", "tr", "ru", "zh"],
+      "required": ["en", "es", "it", "tr", "ru", "zh", "hi", "pl", "ko"],
       "additionalProperties": { "$ref": "#/$defs/steps" }
     },
     "steps": {