"I already finished my lunch."

Translation: 我已经吃完午饭。

[EATandNAP]

It would be more correct grammatically if there was a 了 somewhere. Like 我已经吃完了午饭。Or 我已经吃完午饭了。

[varigby]

No, not really. 了 is more spoken than written.

[FishTsao]

我已经吃完午饭了。