“Watashi wa” is actually not the subject of the sentence in Japanese. It’s what is called a theme, as in: this is what we are going to talk about, the topic.

Example: “Watashi ha hamburger desu”: With regard to “Watashi” = me/I (i.e. concerning “me”), “it is” a hamburger,

Which ends up translated to:

I am a hamburger. Or: For me, it will be a hamburger.

So the “wa” particule (actually “ha”) introduces the theme or topic in the sentence, rather than the subject. Very different from English.

Reference: theme and rheme in Japanese.