The original text here has the bare adverbial form harsh instead of the standard adverbial form ending in -ly (that is, harshly). The 1920 LDS edition supplied the -ly ending. The original Book of Mormon text has a strong preference for the bare adverb when the verb is speak. In the following list, we see that the original text had five examples of the bare adverb plain when the verb was speak:
Thus the original use of harsh in 2 Nephi 33:5 is not surprising. There are also four examples of “to speak plainly” in the original text (see 2 Nephi 25:20 for the list), but there are no examples of “to speak harshly” in the original text.
Summary: Restore the original bare adverb form harsh in 2 Nephi 33:5; such usage is common in the original text when the verb is speak.