feature #4050 [Translation] added logging capability. (aitboudad)

This PR was submitted for the master branch but it was merged into the 2.6 branch instead (closes #4050).

Discussion
----------

[Translation] added logging capability.

| Q             | A
| ------------- | ---
| Doc fix?	 | no
| New docs? | yes(symfony/symfony/pull/10887)
| Applies to | 2.6+
|Fixed tickets |	-

Commits
-------

e8e50fa added logging  to translator.

book/translation.rst

@@ -400,6 +400,15 @@ checks translation resources for several different locales:
 3. If the translation still isn't found, Symfony uses the ``fallback`` configuration
    parameter, which defaults to ``en`` (see `Configuration`_).
 
+.. versionadded:: 2.6
+    The ability to log missing translations was introduced in Symfony 2.6.
+
+.. note::
+
+    When Symfony doesn't find a translation in the given locale, it will 
+    add the missing translation to the log file. For details, 
+    see :ref:`reference-framework-translator-logging`.
+
 .. _book-translation-user-locale:
 
 Handling the User's Locale

reference/configuration/framework.rst

@@ -48,6 +48,10 @@ Configuration
 * `profiler`_
     * `collect`_
     * :ref:`enabled <profiler.enabled>`
+* `translator`_
+    * :ref:`enabled <translator.enabled>`
+    * `fallback`_
+    * `logging`_
 
 secret
 ~~~~~~
@@ -495,6 +499,42 @@ and activate the data collectors by hand::
 
     $profiler->enable();
 
+translator
+~~~~~~~~~~
+
+.. _translator.enabled:
+
+enabled
+.......
+
+**type**: ``boolean`` **default**: ``false``
+
+Whether or not to enable the ``translator`` service in the service container.
+
+fallback
+........
+
+**default**: ``en``
+
+This option is used when the translation key for the current locale wasn't found.
+
+For more details, see :doc:`/book/translation`.
+
+.. _reference-framework-translator-logging:
+
+logging
+.......
+
+.. versionadded:: 2.6
+    The ``logging`` option was introduced in Symfony 2.6.
+
+**default**: ``true`` when the debug mode is enabled, ``false`` otherwise.
+
+When ``true``, a log entry is made whenever the translator cannot find a translation
+for a given key. The logs are made to the ``translation`` channel and at the ``debug``
+for level for keys where there is a translation in the fallback locale and the ``warning``
+level if there is no translation to use at all.
+
 Full default Configuration
 --------------------------
 
@@ -612,6 +652,7 @@ Full default Configuration
             translator:
                 enabled:              false
                 fallback:             en
+                logging:              "%kernel.debug%"
 
             # validation configuration
             validation: