The JEs don't mention the inaka gloss. It's fine but I don't think it should lead.

@@ -16 +15,0 @@
-<gloss>language used in rural areas</gloss>
@@ -18,0 +18 @@
+<gloss>language used in rural areas</gloss>

鄙言	169	67.3%
卑言	0	0.0%
鄙語	82	32.7%

Not this term, but the related 卑語. Only sense [2] would seem to apply.
https://www.jlogos.com/d011/4377315.html
卑語・鄙語【ひご】
①いやしい言葉。
[基本例]「卑（鄙）語を使うものではない。」
[類語]卑言。スラング。
[対語]雅語。
②いなかびた言葉。
[基本例]「原住民の卑（鄙）語は通訳がむずかしい。」

Comments on 卑語[2].  Marked 卑語[2] as [obs] because I don't think it is usable with the sense given here. Not sure if this is [obs], [rare], [dated].   IME won't even build this word(either kanji). The reason that ngrams for 鄙言 beat 卑言 would seem to be that this term has fallen into almost complete disuse.  Google results suggests it is principally vocab trivia.

"language used in rural areas" is wordy, but this is not a term for anyone to use in practice either.  Better that if someone search "vulgar" they don't think to use this.　Given that this is rare and dated, given the kokugo definition "slang" seems out of place. 

jlogos has "スラング" as 類語 on 卑語 for sense [1], but we don't have "slang" there (nor do I think we should).

@@ -14,0 +15,2 @@
+<misc>&rare;</misc>
+<gloss>language used in rural areas</gloss>
@@ -17 +18,0 @@
-<gloss>slang expression</gloss>

GG5

Sorted on hits. Seems not to be quite the same as 鄙諺.

@@ -4,0 +4,3 @@
+<k_ele>
+<keb>鄙言</keb>
+</k_ele>
@@ -12,0 +15,2 @@
+<gloss>vulgar expression</gloss>
+<gloss>vulgarism</gloss>