English term (edited): [the vote ...] shall be the act of the shareholders.

[the vote of a majority of the shareholders present in person or by proxy at a meeting at which a quorum is present]

shall be the act of the shareholders.

Here "act" is used here as a synonym for "decision" - i.e. the result of a vote conducted according to the rules will be "the decision" of the shareholders as a group.

You can safely replace "l'acte" by "la décision", or leave "l'acte".
"la décision" is more straightforward but it should make no difference for the intended audience.

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Note added at 5 hrs (2019-04-24 13:21:38 GMT)
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an act / un acte both in EN and FR can mean two completely different things:

- some doing / action as in "act of commission"

OR

- a written document spelling out some decision, or some laws / regulations as in "Acts of Parliament"

As for the intended meaning in this sentence, you don't need more than the basics of Company law to see which meaning can make any sense:

"the result of a vote ..." is what is the decision of all shareholders as a group THAT is just plain vanilla most usual decision making in a Company having shareholders;
OTOH
the idea that "the vote by shareholders" is what is "teh act" in the sense of "the action they must perform" holds no water.

"le vote ... doit être effectué par les actionnaires" would be just a pointless tautology, and here become total nonsense when combined with "Except as otherwise required by applicable law" as there are no laws whatsoever anywhere that would allow anyone else but the shareholders to vote when a Company needs to take a decision.

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Note added at 2 days 5 mins (2019-04-26 07:46:55 GMT)
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... and here becomes total nonsense ...