minor #15386 [Translation] Improve Translation Providers docs (welcoMattic)

This PR was squashed before being merged into the 5.3 branch.

Discussion
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[Translation] Improve Translation Providers docs

This PR add more precise documentation for Lokalise and PoEditor providers.
Indeed, some settings in those SaaS must be correctly configured in order to make the providers work

Commits
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a2635f3 [Translation] Improve Translation Providers docs

Author: javiereguiluz

translation.rst

@@ -713,6 +713,18 @@ configure the ``providers`` option:
             ],
         ]);
 
+.. tip::
+
+    If you use Lokalise as provider and a locale format following the `ISO 639-1`_ (e.g., "en" or "fr"),
+    you have to set the `Custom Language Name setting`_ in Lokalise for each of your locales,
+    in order to override the default value (which follow the `ISO 639-1`_ succeeded by a sub-code
+    in capital letters that specifies the national variety (e.g., "GB" or "US" according to `ISO 3166-1 alpha-2`_)).
+
+.. tip::
+
+    If you use PoEditor as provider, you have to define a `Default Reference Language`_ in your project,
+    otherwise your translations could not be correctly exported.
+
 Pushing and Pulling Translations
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
@@ -879,3 +891,5 @@ Learn more
 .. _`ISO 639-1`: https://en.wikipedia.org/wiki/List_of_ISO_639-1_codes
 .. _`Translatable Extension`: https://github.com/doctrine-extensions/DoctrineExtensions/blob/main/doc/translatable.md
 .. _`Translatable Behavior`: https://github.com/KnpLabs/DoctrineBehaviors
+.. _`Default Reference Language`: https://poeditor.com/kb/setting-a-default-reference-language
+.. _`Custom Language Name setting`: https://docs.lokalise.com/en/articles/1400492-uploading-files#custom-language-codes