Mahjong soul is probably the best EN client around in terms of helpful interface, providing teaching and info, and having an active playerbase. The theming can be a plus or minus depending on how much you like or dislike gacha games and anime waifus.
If you don't play on Mahjong Soul, other alternatives include:
- Tenhou.net: the big "boring and serious" online mahjong client. Internationally, this was and still is the gold standard of "I have an impressive rank online" if that's the sort of thing you care to brag about. It has a very simple but clear UI and no fluff or transactions to worry about, but not the best for beginners. There's a few plugins for english UI, I like this one: https://chromewebstore.google.com/detail/tenhou-english-ui/jdackjfahmdepglkioclfdhbhdplnana
- Ron2: not free but a really nice online client - I've tried it for a few tournaments and it was one of my favourites.
- Konami Mah-jong Fight Club: Arcade-style client. This was my main for a few years before we got translated EN clients. I really like the polish and "cool" factor, no idea if it's still good anymore.
- Sega Net MJ: The main competitor to the above. Not had much personal experience with it, but it's probably very comparable.
- FFXIV doman mahjong: great if you play the acclaimed MMO FFXIV I suppose. I don't, so I've never tried it, I got a lot of interested players joining my local club after exposure to mahjong through this though.
- Riichi City: A mahjong soul clone. I haven't tried it but it's probably comparable, with maybe less polish and a smaller player base.
- If you have a Switch, nintendo's 51 worldwide games allegedly has a pretty good online mahjong client in it. I can't say I've tried it myself though.
IMO, if you are willing, it's much easier to learn the game in real life, and it builds some good habits that are easy to miss when playing online (like using the sorting and highlighting as a crutch). Nothing tops IRL games, so have a hunt for any local Riichi clubs in your area.