Je n’ai pas parlé français depuis 30 ans.

This is a correct way of saying “I have not spoken French in the last 30 years”, as in “the last time I spoke French was (at least) 30 years ago”. It is not very idiomatic, however. We’d normally use a construct that emphasizes that it has been a long time, similar to “it has been 30 years since I last spoke French” in English.

Cela fait 30 ans que je n’ai pas parlé français.

The sentence

Je ne parle pas français depuis 30 ans.

is definitely not an alternative way of saying this. It’s a different negation of “I have spoken French for the last 30 years”: it means “It hasn’t been 30 years since I first spoke French”, or in other words, “I learnt French less than 30 years ago”. It’s slightly awkward, and a more natural way to phrase this would be

Cela fait moins de 30 ans que je parle français.

I don’t know why Google offers it as a translation, but as you’re aware, automatic translation is extremely unreliable. It’s possible that their engine is combining a correct translation for a positive sentence with a negation rule that doesn’t work here, but this is just conjecture on my part.

Je n’ai pas parlé français depuis trente ans.

means that the last time I spoke French was 30 years ago so correctly translates "I have not spoken French for 30 years" in written French. In spoken French, you more likely hear:

Ça fait trente ans que j’ai pas parlé français.

On the other hand:

Je ne parle pas français depuis trente ans.

is ambiguous as a standalone sentence. It is normally followed by a phrase clarifying the statement.

Here are three possible meanings, the first one being the most common:

Je ne parle pas français depuis trente ans, seulement depuis six mois ! (less than 30 years)

Je ne parle pas français depuis trente ans, je le parle depuis soixante ans ! (more than 30 years)

Je ne parle pas français depuis trente ans, c’est l’italien que je parle depuis trente ans ! (not French but another language)

I confirm that the sentence

Je ne parle pas français depuis 30 ans

is incorrect, for it makes no sense. Still, you might have written:

Je ne parle plus français depuis 30 ans

which means I stopped speaking French 30 years ago.