The phrase "we don't speak the same language" is used metaphorically. It doesn't mean they literally use different tongues, but that their communication has completely broken down.
The lyrics describe a world where one partner calls "day" what the other sees as "night," and where laughter is perceived only as bitterness. Den Milame Idia Glossa
The song tells a poignant story of a couple standing at the "finish line" of their relationship. Despite having shared years of history, they have reached a point where they are effectively strangers living in the same house. The phrase "we don't speak the same language"
Even if they were to live their lives over again, the song suggests the cycle would repeat: "you would hurt, I would regret". Cultural Impact I would regret". Cultural Impact