It isn't the fault of Google Translate guys, "Be My Valentine" is basically a confession phrase unique for Valentine day. Japan obviously doesn't have similar culture as their valentine day is just typical chocolate gifting (capitalism hoe) and special day for couple is on 24 December (christmas eve) instead of 14 February.
Not saying there aren't confession and dick thrusting on Valentine day in Japan, it's just not as special as on western countries. Also they confess using normal word and not via card.