I write in multiple languages daily on Linux, including English, Russian, and Chinese. Switching keyboards (at least with gnome) is a simple super-space.
While in my default (English) layout, it is easy enough to add in accents other characters using the compose key (right alt). So right-alt+'+a = á or right-alt+"+u = ü. I much prefer this over the long press as I can do it quickly and seamlessly without having to wait on feedback. Granted, it is not as discoverable, but once you are comfortable, it in my opinion is a better system.
While in my default (English) layout, it is easy enough to add in accents other characters using the compose key (right alt). So right-alt+'+a = á or right-alt+"+u = ü. I much prefer this over the long press as I can do it quickly and seamlessly without having to wait on feedback. Granted, it is not as discoverable, but once you are comfortable, it in my opinion is a better system.