[Translation] Add a tip about Crowdin language mapping

Author: andrii-bodnar

translation.rst

@@ -706,6 +706,13 @@ configure the ``providers`` option:
     Also make the locale _names_ in Phrase should be as defined in RFC4646 (e.g. pt-BR rather than pt_BR).
     Not doing so will result in Phrase creating a new locale for the imported keys.
 
+.. tip::
+
+    If you use Crowdin as a provider and some of your locales are different from
+    the `Crowdin Language Codes`_, you have to set the `Custom Language Codes`_ in the Crowdin project
+    for each of your locales, in order to override the default value. You need to select the
+    "locale" placeholder and specify the custom code in the "Custom Code" field.
+
 .. tip::
 
     If you use Lokalise as a provider and a locale format following the `ISO
@@ -1546,3 +1553,5 @@ Learn more
 .. _`Symfony Demo`: https://github.com/symfony/demo
 .. _`Identification via User-Agent`: https://developers.phrase.com/api/#overview--identification-via-user-agent
 .. _`Phrase Tag Bundle`: https://github.com/wickedOne/phrase-tag-bundle
+.. _`Crowdin Language Codes`: https://developer.crowdin.com/language-codes//
+.. _`Custom Language Codes`: https://support.crowdin.com/project-settings/#languages/