readme: add the command to only update a specific language

Author: OniriCorpe

tools/readme_generator/README.md

@@ -55,10 +55,12 @@ source venv/bin/activate
 # Extract the english sentences from the code, needed if you modified it
 pybabel extract --ignore-dirs venv -F babel.cfg -o messages.pot .
 
-# If working on a new locale: initialize it (in this example: fr)
+# If working on a new locale: initialize it: (in this example: fr)
 pybabel init -i messages.pot -d translations -l fr
 # Otherwise, update the existing .po:
 pybabel update -i messages.pot -d translations
+# To update only a specific language: (in this example: fr)
+pybabel update -i messages.pot -d translations -l fr
 
 # ... translate stuff in translations/<lang>/LC_MESSAGES/messages.po
 # re-run the 'update' command to let Babel properly format the text