Translation cards in the play area is a disruption. I know. I was there judging. Judges rely on the ability to quickly scan the playing areas as they roam. Maybe for you it doesn't cause disruptions, but for some, it does - both players AND judges.
The problem in life is that sometimes what affects one person might not directly affect others.
Finally, the "only time one would NEED a translation" is EVERYTIME a player plays a unique foreign-language card. Technically, the player MUST provide a translation, regardless of whether it's asked for or not, unless the opponent declines. So, everytime you play a foreign-language card, if the translation is not visible, you MUST provide it, or let your opponent decline. A dubious tactic would be saying something like, "You know what this card does, right? Good." After which you DON'T provide the translation, assuming your opponent doesn't want to see. That could be construed as an intimidation factor against novice and/or younger players.