i18n: document not to use old-style (%) string formatting

electrum/i18n.py

@@ -30,6 +30,11 @@ LOCALE_DIR = os.path.join(os.path.dirname(__file__), 'locale')
 language = gettext.translation('electrum', LOCALE_DIR, fallback=True)
 
 
+# note: do not use old-style (%) formatting inside translations,
+#       as syntactically incorrectly translated strings would raise exceptions (see #3237).
+#       e.g. consider  _("Connected to %d nodes.") % n
+#                      >>> "Connecté aux noeuds" % n
+#                      TypeError: not all arguments converted during string formatting
 # note: f-strings cannot be translated! see https://stackoverflow.com/q/49797658
 #       So this does not work:   _(f"My name: {name}")
 #       instead use .format:     _("My name: {}").format(name)