Added some documentation about translations lifecycle

TRANSLATORS.rst

 Translating Funkwhale
 =====================
 
+A step-by-step, beginner friendly guide is available at https://contribute.funkwhale.audio/guides/translate/
+
 Thank you for reading this! If you want to help translate Funkwhale,
 you found the proper place :)
 
@@ -21,8 +23,41 @@ Respecting those guidelines is mandatory if you want your translation to be incl
 
 - Use gender-neutral language and wording
 
+Submitting a new language
+-------------------------
+
+1. Pull the latest version of ``develop``
+2. Create a new branch, e.g ``git checkout -b translations-new-fr-ca``
+3. Add your new language code and name in ``front/src/locales.js``. Use the native language name, as it is what appears in the UI selector.
+4. Create the ``po`` file from template:
+
+.. code-block:: shell
+
+    export LOCALE=fr_CA  # replace with your actual locale code
+    mkdir -p front/locales/$LOCALE/LC_MESSAGES
+    msginit --no-wrap --no-translator --locale=$LOCALE --input=front/locales/app.pot --output-file=front/locales/$LOCALE/LC_MESSAGES/app.po
+
+5. Then commit your changes, push, and submit a pull request on the ``develop`` branch
+
 Requesting a new language
 -------------------------
 
-If you'd like to see a new language in Funkwhale, please open an issue here:
+If you cannot submit a new language yourself, you can request it by opening an issue here:
 https://dev.funkwhale.audio/funkwhale/funkwhale/issues
+
+Extracting messages from source
+-------------------------------
+
+We offer a script to update existing ``po`` and ``pot`` files with new translations
+from the source code. This action should be run regularly, and in particular before
+lots of translation work is expected (e.g a few weeks before a new release), or when
+the UI code changes a lot. Otherwise, translators end up translating some obsolete messages,
+or not translationg new messages.
+
+1. `Lock the translations on weblate <https://translate.funkwhale.audio/projects/funkwhale/front/#repository>`_ (``Lock`` button in the sidebar). This will prevent translators from working, and help prevent potential conflicts in the source code
+2. `Commit and push changes from weblate <https://translate.funkwhale.audio/projects/funkwhale/front/#repository>`_ (``Commit`` and ``Push`` buttons in the sidebar)
+3. Pull ``develop`` in your local git repository to ensure you have the latest version of the translations
+4. Create a dedicated branch with ``git checkout -b translations-integration``
+5. Extract the translations with ``cd front && ./scripts/i18n-extract.sh``. This will update all ``po`` files as necessary
+6. Review, commit and push the changes, then open a merge request on the ``develop`` branch
+7. When the MR is merged, `Unlock the translations on weblate <https://translate.funkwhale.audio/projects/funkwhale/front/#repository>`_ (``Unlock`` button in the sidebar).