MD Composer Extras
This theme component adds few formatting options in the composer : underline, align center, align right, justify and strikethrough. You can customize the text of each button in the settings, making it compatible with every languages. Exist in rtl version (change align right to align left).
Preview the theme component on the theme creator forum
This theme component is an alternative of the Discourse Formatting Toolbar, I don’t recommend using both.
I recommend the plugin, but if you don’t want to (or can’t) install a plugin, this theme component might add enough options in the composer for you.
Also, to adapt the the component in mobile devices, I hid some buttons : bullet list, list and justify. They seemed the less helpful in this device.
For RTL languages forums, here is the rtl version : https://github.com/iunctis/md-composer-extras-rtl
Let me know if there is some issue !
I used some work done by @Johani in his theme components to add translations of the elements, and auto-integration of fa5 icons.