The translation should be controlled by receiver rather than sender (of messages).
With the current design, if a group space has members from 4 different languages, then, the sender needs to invoke 4 different translator Bots (@ToJapanese, @ToFrench, etc.), for each message. This is inefficient and actually impedes collaboration.
On the other hand, if the receiver can set up preferred language, then, sender will need to post each message only once- the receiver will see the message on his/her screen translated to their individual preferred language.