Using Composer is a great way of organizing your WordPress project, with WordPress itself, plugins and themes declared as dependencies. However, an issue I’ve often seen is how you install translation files and keep them updated.
WordPress translations are compiled from human-readable PO-files into machine optimized MO-files, but still takes a lot of overhead to load. If you only could cache the translation load time, you would save a lot of page load time. We can easily cut 90% of our WordPress translations loading time.
Since I translate a lot of WordPress themes and plugins, I sometimes come across plugins who try to be clever with their translations. This tends to not work so well in reality.