How do you say I love you in Japanese Wouldent it just be Watashi wa ai anata?

No, that would be Word to Word translation, regardless of the language's unique grammar.

[If you don't need the summarized grammatical explanation just skip to the last line.]

1) In Japanese 'pronouns' (I/He/Me/Their/Them/etc) are omitted in direct speech and most of the time in general.

2) Also there are particles in Japanese that are non-existent to English, such as 'accusative' (object indicator 'wo') and the 'topic marker' ('WA') and another doing both WA and wo's job partially, 'ga'.

3) Just because 'love' as a noun would be 'ai', it doesn't mean that like in English, the verb form is the same too. Japanese has its own verb patterns and conjugations.

So, literal translation for that would be:

'Watashi (I) WA (particle) anata (you) wo (accusative) ai (love) shiteimasu (present continuous/progressive of the verb 'suru' which affixed to 'ai' make a verb out of it)

But that would be kind of over-grammatical, even though correct it is not used in daily speech. Simply 'Aishite imasu' for polite form and 'Aishiteru' for informal form are enough and what is common usage.