Enhance CGL appendix for XLIFF files

Documentation/Appendix/Cgl.rst

@@ -15,6 +15,7 @@ section of the "TYPO3 Explained" manual (formerly known as "Core API").
 
 This is just a very brief overview.
 
+..  _appendix-cgl-php:
 
 PHP
 ===
@@ -31,19 +32,30 @@ to configure the correct coding style.
 The Appendix contains information on scripts to check / fix coding
 guideline issues: :ref:`cgl-fix-my-commit`
 
+Most PHP coding guidelines can be automatically applied with the command:
+
+..  code-block:: bash
+
+    Build/Scripts/runTests.sh -s cgl
+
+..  _appendix-cgl-javascript:
+
 JavaScript
 ==========
 
 The following rules should be applied for JavaScript:
 `Airbnb JavaScript Style Guide <https://github.com/airbnb/javascript>`__
 
+..  _appendix-cgl-typescript:
 
 TypeScript
 ==========
 
 The following rules should be applied for TypeScript:
 `Excel Micro TypeScript Style Guide <https://github.com/excelmicro/typescript>`__.
 
+..  _appendix-cgl-xliff:
+
 Xliff files
 ===========
 
@@ -52,3 +64,37 @@ in files with the ending *.xlf*. While no tabs are allowed to indent
 in PHP files, you should edit Xliff files using tabs.
 Please also check :ref:`common-review-checks-xlf` for Xliff-specific things
 to pay attention to.
+
+..  _appendix-cgl-xliff-filename:
+
+XLIFF file naming
+-----------------
+
+XLIFF files have historically been named with file names starting with
+`locallang_`. Newly introduced XLIFF files should drop the `locallang_`
+prefix and only be named according to their purpose, for example
+`clipboard.xlf` or `db/tt_content.xlf`. Newly introduced file
+names should use **snake_case**.
+
+TCA-related XLIFF files should either be called `db.xlf` or be stored in a
+folder named `db`, for example `db/tt_content.xlf`.
+
+..  _appendix-cgl-xliff-reference:
+
+Label reference naming
+----------------------
+
+Newly introduced label references should use *snake case*. Different parts of a
+label reference can be separated by dots, for example:
+
+..  code-block:: xml
+
+    <trans-unit id="my_field.title">
+        <source>My title</source>
+    </trans-unit>
+    <trans-unit id="my_field.description">
+        <source>My description</source>
+    </trans-unit>
+
+Existing labels should not be mass-changed to snake case, as each changed label
+must be retranslated into all languages.