In this verse, David asks if the Lord will fail the plans of his enemies (cf. 2 Samuel 17:14). This verse ends with they will be exalted.
Translators and commentators explain this in different ways. Some connect it with the verse that follows, so that it reads: They exalt (= choose) the head of those who surround me.
Others translate Let the plans fail when they rise against me.
Or: Thwart their plans, otherwise they will be exalted (become proud).
All these translations are linguistically possible, but the last one gives the best possibility, I think. For you often see that unbelievers become proud when their plans work. If all is well, believers react with thankfulness when plans succeed.
9 Eternel, n'accomplis pas les désirs du méchant,Ne laisse pas réussir ses projets, de peur qu'il ne s'en glorifie! Pause.