Historically, there are two adverbial endings in English, -ly and -e. The latter was originally pronounced as a schwa in Middle English but became silent by the end of the Middle English period, resulting in what might be called the bare (or flat) adverb. The result was that there was no difference anymore between this adverbial form and the corresponding adjective. The tendency in the modern language (supported explicitly by prescriptivist dicta) has been to favor the alternative adverbial form ending in -ly because this alternative form is distinguishable from the adjective. Nonetheless, there are expressions in modern English where the earlier adverbial form that originally ended in -e has been maintained, as in “he ran fast” and “she worked hard”. And of course, there is variability in speech, so that “he drove slow” and “he drove slowly” are both heard. (For further discussion and examples, see flat adverbs in Merriam Webster’s Dictionary of English Usage.)
In the Book of Mormon text, this kind of variability between the two different adverbial forms is found with the adverb plain(ly), providing it occurs with the verb speak. Besides the example listed here in 2 Nephi 25:20, there are four others that originally read plain rather than plainly:
For all five cases of the original adverbial plain, the current LDS text has the standard adverbial form plainly. The 1920 edition is responsible for making the change in four out of the five cases; the example in Alma 5:43 was first changed in 1907 (in the third printing of the Chicago missionary edition and in the 1907 vest-pocket edition). On the other hand, the RLDS text has consistently maintained the original plain.
It should be noted that there are four cases of the standard “to speak plainly” in the earliest text. Of course, none of these have ever been edited:
With verbs other than speak, the Book of Mormon text has only plainly:
The critical text will retain the original adverbial form (either the -ly form or the bare adverb) according to the reading of the earliest textual sources.
Summary: Maintain each case of “to speak plain” when it is supported by the earliest sources, here in 2 Nephi 25:20 as well as in 2 Nephi 25:28, Mosiah 2:40, Alma 5:43, and Helaman 8:4.