I'd suggest that instead of having punishments correlating strictly to the number of prior offences committed, you categorize the offence based off of severity so the punishment fits better. So, for a chat related offence, the punishment should also be chat related. I don't think players should ever be account banned as a sanction for a chat infringement. For example, players who speak in banned languages or use profanity in global should be verbally warned or muted for 1 or 2 days. If they continue to make the same offences and aren't learning, the duration of mutes should increase, but an account ban should be off limits for chat offences.
If players are reported for swearing in clan chat by another clan member, I think that should also be taken into special consideration. Clan chat is expected to be more private than global chat, as what you say there isn't accessible to everyone on the server. Muting people for chat related offences exclusively made in clan chat would seem unwarranted, as the person in clan who is 'offended' could easily leave that clan if they aren't comfortable there and join another, or just raise that issue to the clan to get it addressed. But let's be honest, in all likelihood, this type of clan chat reporter is the type to use reports as weapons. Giving clan chat reports a special consideration would be a way to lessen the impact of that type of unproductive player. In this situation, I think a verbal warning from a GM would be enough. I don't think people should be globally muted for something they say in clan chat.
Players who are actually abusive/harassing (derogatory slurs, racism, sexism, etc.) towards other players in global chat should be globally muted for a longer duration than they would be for a lesser chat offence. Account bans should be strictly reserved for serious game offences, such as using bots or hacking, and this should have its own separate set of tiers of ban durations based on the type of game offence committed.
It's also worth noting that every player has an in-game mute function, and its use should be encouraged more. In this regard, perhaps the report structure can be changed so that instead of globally muting players for less serious things like 'baka', the GMs are instead recommended to open a private message with the reporter and suggest muting the players they feel offended by.
These are just some suggestions and the details could be ironed out later, but they would go a long way to making the chat more active and productive, and less weaponized.