@Gargron @numimyon So it is possible, but the on;y reason you don't include that is because you think the webapp gets to many options. But this is an option for the compose window and one extra language icon right of CW wouldn't be clutter the UI in my opinion
A solution would be to add the option 'manually select language' in the posting language settings. When enabled a language icon will appear below the compose window (with the interface language preselected). Than is everyone happy.