Linux supports Unicode usernames. However, bidirectional rendering is application-specific. I have not tried this myself, so I’m not sure what happens. Also, I’m not sure whether the bidirectional markers are canonicalized away, so the input method likely matters too.

If you support unicode you can d̸͍̳̣̱̼̞̠̝͚̽͑̀̾̈́̎̇̀͂̆̀̚͟͜ͅò̸̧̻̩̲͈̭͎͎̗̝̫͉̓̕͠ ̴̝̜̻̝̹̘̺͜͠a̶̛̱͙͙̭̟͍͍̟̫͎͉͐͌͂̎̇͊̈́͋̇̔̈̏̚l̵̢̜͉̩͍̯̯̤͔̪̲̟͈̅̌̉̏̾͑̓͂͆̀̐͂͟͟͡ļ̷̜̥͍͓͉͈̮̻̰̘̹̣̒͌̒̑̅͂͐̏̾̓̀̽͟͝͡ ̷̡̛͚̻̪͈͕̭̪͎̪̎͑́̆͑͂̉͑͜͠🄺🄸🄽🄳 🄾🄵 𝔴𝔢𝔦𝔯𝔡 𝓈𝓉𝓊𝒻𝒻.

Yes, if you support Unicode fully you’ll support Arabic automatically. There are RTL code points that will switch text direction automatically. Browsers support this at the html and css level. There’s a bunch of different ways.

yes

If you won’t provide any details no one will be able to help you.