i18n: Add Crowdin config & CI

Need to specify `languages_mapping` explicitly, because Crowdin doesn't provide a lowercase locale with underscores, as required by Minecraft.

.github/workflows/crowdin-download-translations.yaml

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+name: Crowdin (Download Translations)
+
+on:
+  workflow_dispatch: # Allow running manually
+  schedule:
+    # https://crontab.guru
+    - cron: '0 */12 * * *' # Every 12 hours
+
+# Queue this workflow after others in the 'crowdin' group, to avoid concurrency issues.
+concurrency:
+  group: crowdin
+
+permissions:
+  contents: write
+  pull-requests: write
+
+jobs:
+  download_translations:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Download to PR
+        uses: crowdin/github-action@v1
+        with:
+          upload_sources: false
+          upload_translations: false
+          download_translations: true
+          skip_untranslated_strings: true
+          crowdin_branch_name: ${{ github.ref_name }}
+
+          create_pull_request: true
+          pull_request_title: Crowdin translations for ${{ github.ref_name }}
+          pull_request_body: |
+            New pull request with translations from [Crowdin](https://crowdin.com/project/freecam).
+            
+            Powered by [Crowdin's GitHub Action](https://github.com/crowdin/github-action).
+          pull_request_labels: enhancement, i18n
+          pull_request_base_branch_name: ${{ github.ref_name }}
+          localization_branch_name: i18n/${{ github.ref_name }}
+          commit_message: |
+            i18n: New translations from Crowdin
+            
+            [skip ci]
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+          CROWDIN_PROJECT_ID: ${{ secrets.CROWDIN_PROJECT_ID }}
+          CROWDIN_PERSONAL_TOKEN: ${{ secrets.CROWDIN_PERSONAL_TOKEN }}

.github/workflows/crowdin-upload-sources.yaml

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+name: Crowdin (Upload Sources)
+
+on:
+  workflow_dispatch: # Allow running manually
+  push: # Run when en_us is modified on the main branch
+    branches:
+      - main
+    paths:
+      - '**/lang/en_us.json'
+
+# Queue this workflow after others in the 'crowdin' group, to avoid concurrency issues.
+concurrency:
+  group: crowdin
+
+jobs:
+  upload_sources:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Upload Sources
+        uses: crowdin/github-action@v1
+        with:
+          upload_sources: true
+          upload_translations: false
+          download_translations: false
+          crowdin_branch_name: ${{ github.ref_name }}
+        env:
+          CROWDIN_PROJECT_ID: ${{ secrets.CROWDIN_PROJECT_ID }}
+          CROWDIN_PERSONAL_TOKEN: ${{ secrets.CROWDIN_PERSONAL_TOKEN }}

.github/workflows/crowdin-upload-translations.yaml

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+name: Crowdin (Upload Translations)
+
+on:
+  workflow_dispatch: # Allow running manually
+## (Disable automatic runs for now)
+#  push: # Run when a non-en_us lang file is modified on the main branch
+#    branches:
+#      - main
+#    paths:
+#      - '**/lang/*.json'
+#      - '!**/lang/en_us.json'
+
+# Queue this workflow after others in the 'crowdin' group, to avoid concurrency issues.
+concurrency:
+  group: crowdin
+
+jobs:
+  upload_translations:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Upload Translations
+        uses: crowdin/github-action@v1
+        with:
+          upload_sources: false
+          upload_translations: true
+          download_translations: false
+          crowdin_branch_name: ${{ github.ref_name }}
+        env:
+          CROWDIN_PROJECT_ID: ${{ secrets.CROWDIN_PROJECT_ID }}
+          CROWDIN_PERSONAL_TOKEN: ${{ secrets.CROWDIN_PERSONAL_TOKEN }}

crowdin.yaml

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+project_id_env: CROWDIN_PROJECT_ID
+api_token_env: CROWDIN_PERSONAL_TOKEN
+preserve_hierarchy: true
+
+files:
+  - source: '/**/lang/en_us.json'
+    translation: '/**/lang/%locale%.json'
+    languages_mapping:
+      locale:
+        # Valid keys: https://developer.crowdin.com/language-codes
+        # Valid values ("in-game" locale column): https://minecraft.wiki/w/Language
+        it: it_it
+        ja: ja_jp
+        zh-CN: zh_cn