Multibyte characters are 100% supported, but only if they are available in the chosen font. The fallback fonts feature will resolve this issue for you.
> Possible bug: on macOS when I minimize Alacritty, and I put it on focus again, it tries to select text. Strangely, not always.
Definitely a bug, and it's one that I knowingly shipped with (usually I just resize my terminal once and then it's static). The events triggering selection seem to work slightly different on macOS than Linux. The issue should be easy to resolve, but this comes down to making time.