feat: configurable report language (default zh-CN)

Add providers.yaml `language` / Action input `language`; prompts and
published summary/inline honor it. Soften fail-closed messaging when
models 429; prefer cohere/nvidia in the first-3 parallel set for dogfood.

action.yml

@@ -21,6 +21,12 @@ inputs:
       (or bare names unique in config). Empty = providers.yaml lists.
     required: false
     default: ""
+  language:
+    description: >
+      Report language for model prose + published summary/inline comments
+      (e.g. zh-CN, en). Empty = providers.yaml `language`, else en.
+    required: false
+    default: ""
   skip-draft:
     description: Skip draft PRs
     required: false
@@ -102,6 +108,7 @@ runs:
         GITHUB_TOKEN: ${{ inputs.github-token }}
         CONFIG_DIR: ${{ inputs.config-dir }}
         GATEKEEP_MODELS: ${{ inputs.models }}
+        GATEKEEP_LANGUAGE: ${{ inputs.language }}
         SKIP_DRAFT: ${{ inputs.skip-draft }}
         GATEKEEP_ROOT: ${{ github.workspace }}
         GATEKEEP_ACTION: ${{ github.action_path }}