applicationwindow: add note about gettext compatibility

62f7395c fleshed out the xml for an gtk_builder_new_from_string example.

It also marked labels as translatable, but this doesn't have support by gettext (see https://savannah.gnu.org/bugs/?64481).

The only other major usage of gtk_builder_new_from_string with translatable I found was in GIMP, and there the translatability was solved by moving to using gtk_builder_new_from_resource and a separate .ui file instead. gimp@637385c7

Since 62f7395c does not mention translatability, I wonder if the translatable were added by mistake since it doesn't work with gettext anyway.