Why would Joseph Smith be so confident to give the interpretation of the word Rabbanah?

In Alma 18:13 we read of the word “Rabbanah” and its interpretation:

And one of the king’s servants said unto him, Rabbanah, which is, being interpreted, powerful or great king, considering their kings to be powerful; and thus he said unto him: Rabbanah, the king desireth thee to stay.

Why would Joseph include the interpretation if he didn’t know any other languages at the time? The word Rabbanah is a direct hit for Joseph Smith, as explained by Matthew Bowen:

The servants of Lamoni, however, ask Ammon to stay calling him by the name “Rabbanah” which is glossed as “great or powerful king,” a title evidently built from the Semitic root *rbb/rby, which denotes “greatness” or “muchness” (rabb is still the word for [divine] “Lord” in modern Arabic).
– Matthew L. Bowen – Father Is a Man: The Remarkable Mention of the Name Abish in Alma 19:16 and Its Narrative Context

Why would Joseph take an unnecessary chance by giving the interpretation of words in the Book of Mormon?

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