וַיֹּ֣אמְרוּ לוֹ֩ אֱמָר־נָ֨א שִׁבֹּ֜לֶת וַיֹּ֣אמֶר סִבֹּ֗לֶת וְלֹ֤א יָכִין֙ לְדַבֵּ֣ר כֵּ֔ן וַיֹּאחֲז֣וּ אוֹת֔וֹ וַיִּשְׁחָט֖וּהוּ אֶל־מַעְבְּר֣וֹת הַיַּרְדֵּ֑ן וַיִּפֹּ֞ל בָּעֵ֤ת הַהִיא֙ מֵֽאֶפְרַ֔יִם אַרְבָּעִ֥ים וּשְׁנַ֖יִם אָֽלֶף׃
ay sinasabi nila sa kanya, “Sabihin mo nga: Shibolet.” At sinasabi niya, “Sibolet,” sapagkat hindi niya magawang bigkasin ito nang wasto; at hinuhuli nila siya at pinapatay sa mga tawiran ng Jordan. At nabuwal nang panahong iyon mula sa Efraim ang apatnapu't dalawang libo.
Rashi
Say Shiboles please. May I cross this river flow?1Shiboles” is translated as “flow”.
Radak
Shibboleth (שִׁבֹּלֶת) – Just as they would test them with this word, so too they would test them with ever word that has a שׁ (shin) in it, and the Ephrathites would read it as שׂ (sin), and the text chose one of the words. And some say that he chose the word "שבולת" (shibboleth) because there was a flowing stream (shibboleth nahar) in the place where the Ephraimites were crossing, who would stutter on the letter שׁ (shin). Maybe the climate of their land was a factor! This is like the people of France, who do not understand how to read שׁ (shin) and read it like a weak תֿ (thaw).
Rashi
They were not proficient in proper pronunciation. They were speech deficient.