The first verb in verse 5 can be understood as “they strengthen (for) themselves,” which Good News Translation has rendered by “They encourage each other,” and New Jerusalem Bible “They support each other” (see also New International Version, Bible en français courant, Biblia Dios Habla Hoy). Another possibility is Revised Standard Version They hold fast, or else “they persist.”

In verse 5b the Hebrew “they talk about to hide their snares” means that they discuss how to place their traps in such a way that the intended victim does not see them and so will be caught in them.

In verse 5c the question in Hebrew, “Who can see them?” can be understood as an indirect question: “they ask who can see them,” that is, who can see the people who are laying the traps, not the traps (so New English Bible). But the Hebrew interrogative pronoun generally introduces a direct question, and for this reason Revised Standard Version prefers to emend to Who can see us? This is more in line with the usual language in such cases (see 10.11, 13; 59.7).5-6 Hebrew Old Testament Text Project says the verb raʾah means “to notice, to discover somebody,” so that the question means “Who will see them?” It appears, therefore, that “them” refers to the people, not to the traps. These wicked people think that no one, including God, sees what they are doing.

Verse 6 is particularly difficult as to both the form and the meaning of the Hebrew text. Oesterley does not translate the verse, with the comment that the various drastic emendations proposed “do not inspire confidence.” Anderson makes a similar statement: “none of the suggested emendations is really convincing.”
(1) The first line in Hebrew seems to mean “they devise evil plans,” a translation of the verb meaning “search out, devise” (see its use in 77.7).
(2) The second line in Hebrew seems to mean “we completed a well-planned plot.” The verb in the Masoretic text is tamam, “be complete, finished”; the noun, which occurs only here in the Old Testament, is related to the verb used in the first line and is taken to mean “plot”; the adjective appears to be a passive of the same verb. But some take the Masoretic text verb to be the equivalent of the third person plural “they completed”; and many Hebrew manuscripts have the similar Hebrew verb taman, “to hide” (see 9.15).
(3) The third line in Hebrew is “and the inner part of a man and the heart (are) deep.”

Some take the first line as a question and still part of what the wicked say; so Revised Standard Version, New Jerusalem Bible (“who … will penetrate our secrets?”). Good News Translation takes it as a statement (likewise New English Bible, New American Bible, Traduction œcuménique de la Bible) and takes the verb to mean “devise”; see New International Version “they plot” and Bible en français courant “they imagine.”

In the second line the verb in the Masoretic text is the first person plural, so Good News Translation translates it as a statement made by the wicked men, “We have planned a perfect crime” (so Traduction œcuménique de la Bible “We have planned well our plot”; see also Bible en français courant, New International Version). But many prefer the verb “hide,” which is found in many Hebrew manuscripts, and translate “they conceal their cunning plot” (see New American Bible). New Jerusalem Bible takes lines b and c to be the psalmist’s reply to the wicked men’s question in line a (Who can search out our crimes?) and translates “He will do that, he who penetrates human nature to its depths, the depths of the heart.”

The third line is taken by Revised Standard Version and Good News Translation to be the psalmist’s comment; but Traduction œcuménique de la Bible takes it to be still a part of what the wicked themselves say (“in man’s depths, the heart is impenetrable”).

New Jerusalem Bible translates the whole verse (reading the verb “to hide” in line b): “Let the wrongdoings they have concealed, each one inside him, his secret thoughts, be wholly exposed” (with a marginal note). New English Bible has “they hatch their secret plans with skill and cunning, with evil purpose and deep design” (with two textual footnotes). Weiser has “They devise crimes, keep secret their plot; for a man’s bosom and heart are deep.” Biblia Dios Habla Hoy follows New Jerusalem Bible, and Dahood’s translation is similar to that of New Jerusalem Bible.5-6 Hebrew Old Testament Text Project has three separate notes for this verse and concludes: This difficult v. may be understood in two ways: 1. “they plotted crimes: ‘we achieved a plotted plot: to man’s inward mind secret plans (lit. a deep heart)!’ ” The v. contains a quotation of the wicked men’s words which are expressed in the manner of a proverb. 2. “they plotted crimes: ‘we have achieved a plotted plot: but man’s inward mind and his heart deep!’ ” This second interpretation is less probable, for it does not follow the phrase division of the MT.

The translator must make a decision, in the knowledge that any translation is itself conjectural.

Quoted with permission from Bratcher, Robert G. and Reyburn, William D. A Handbook on the Book of Psalms. (UBS Helps for Translators). New York: UBS, 1991. For this and other handbooks for translators see here .