Here’s the Greek:
5 οὐκ ἔσται ὁ λόγος σου ψευδής, οὐ κενός, ἀλλὰ μεμεστωμένος πράξει.
Here’s my translation [at present]:
5 Your speech should not be false or empty, but filled with action.
The key here for me, given the presence of αλλα, is the contrast between κενός and μεμεστωμένος, which is why I translate the portion following αλλα as “filled with action”. In my view, αλλα marks contrast and also indicates the portion following the αλλα corrects or replaces the portion before it. The above is an instance of correction, what you say should be backed up by what you do.
The basic idea of μεστοω [according to BDAG, anyway], is that the speech/word is “made full” by one’s action. This is where the idea of completed/fulfilled/confirmed comes from. It’s as if one’s actions verify that his words/speech is worthy of being listened to. The idea is almost as if one’s actions verify that one’s speech it true.
Given the innate contrast, it seems better (at least to me) to play off of that contrast by using “filled” to translate μεμεστωμένος and using “with action” to translate πράξει.
Here’s how others I know of translate the verse:
Thy speech shall not be false nor vain, but completed in action. (Lake)
Thy word shall not be false or empty, but fulfilled by action. (Lightfoot)
Your word must not be false or meaningless, but confirmed by action. (Holmes)
Your word will not be false or empty, but will be fulfilled in action. (Varner, who follows the Greek text of Rordorf & Tuilier’s 1978 edition, which was republished in 1998 as part of Sources Christiennes)
Your word shall not be false or empty [but shall be fulfilled by deed]. (Niederwimmer, he sees the brackets as a later addition)
Your word must not be empty or false. (Ehrman, who apparently takes the reading of the Apostolic Confessions over Heirosolymitaunus [as does Niederwimmer, apparently]; note his Greek text is a modified version of Bihlmeyer)
These guys all have (or had while alive) more Greek in the tip of their left pinkie toe than I’ve got in the entirety of my being, but given contrast marked by αλλα and the notion of correction, I still think I like “filled with” better; though “fulfilled by” is a pretty close second (that I could actually be convinced of).