In the 75th or 76th year of the judges, Nephi son of Helaman says in a prayer that the band of Gadianton has been eradicated. I have added in brackets the implied referents of the third-person pronouns and possessives, noting that one of them is ambiguous.
O Lord, behold this people repenteth; and they [the people] have swept away the band of Gadianton from amongst them [the people] insomuch that they [the Gadiantons] have become extinct, and they [who?] have concealed their [the Gadiantons'] secret plans in the earth. Now, O Lord, because of this their [the peoples'] humility wilt thou turn away thine anger . . . (Hel. 11:10-11).
The first possible reading is that the people have demonstrated their humility by (1) repenting, (2) wiping out the Gadianton band, and (3) concealing the Gadiantons' secret plans in the earth.
The second possible reading is that the people have been so successful in sweeping away the Gadianton band that that the Gadiantons have (1) become extinct and (2) been forced to conceal their secret plans in the earth.
The first is the most natural reading for a couple of reasons. First, "their humility" definitely refers to the humility of the people, but that reading would be difficult if the three instances of they/their preceding it all referred to the Gadiantons. Second, it is unnatural to say that the Gadiantons "have become extinct and" done something else; they obviously would have had to conceal their plans before they became extinct, and so it would be more natural to say that first in the sentence. If the meaning of the passage is that, although the Gadiantons' themselves have become extinct, their plans are still hidden somewhere in the earth, we would expect a but rather than an and. Because of its naturalness, I assume many readers of the Book of Mormon (including myself until very recently) take the first reading for granted and don't even notice the ambiguity.
The official Chinese translation of the Book of Mormon removes the ambiguity and makes the first reading the only one possible. It says, essentially, that the people have "repented, wiped out the Gadiantons, and concealed their secret plans." The subject of all these verbs can only be "this people."
However, the Russian translators made the opposite call: "они скрыли свои тайные планы" can only mean "they hid their own secret plans." If anyone other than the Gadiantons themselves had hidden the plans, the correct possessive would be их rather than the reflexive свои.
Despite what I have said about the naturalness of the first reading, I can see the Russians' point of view, too. Why would the people who eradicated the Gadiantons themselves carefully conceal the Gadiantons' secret plans instead of just destroying them? Years later, people dig up the plans and use them to start a Gadianton revival. For that even to have been possible, the plans must have been intentionally buried in such a way that they would be preserved -- something analogous to Moroni writing on gold plates and burying them in a stone box. It's hard to imagine any motive for the enemies of the Gadiantons to do that.
If the Gadiantons themselves buried the plans so that they would be preserved for future generations, that raises questions, too. It suggests that the "secret plans" were not just a playbook for criminal mischief but were seen by the Gadiantons as having some almost religious value.
I'm still trying to work out possible understandings of the whole "secret plans" plot point. I just want to document this ambiguity in the text first and see what other people think.