Imo... They should use the google translate API and add it to the client.

E.g.

Player A has set in their config that they speak english
Player B has set in their config that they speak japanese

Player A and Player B enter a group,

Player A has auto translate turned on so when player b types in chat, player a's client automatically google translate what they said and displays the english translation.

Though this should be something users can turn off on their end if it makes them lag.