When do you say "Pole"?
When would you say "Pole"Is it for a bigger problem, like someone dying, or is it small like "Oh I bumped into you"?

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I have also heard "pole, pole" used to mean "slowly, slowly" when used twice in a row.

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the word ''pole'' simply means ''sorry/i am sorry'' 
pole can be used for both big & small problems.

from the native speaker

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I'm not a native speaker, so it's just my opinion.
I used to stay in Kenya, and I heard people saying this word so many times, almost everyday.
They say "pole" when someone has a problem whichever big or small.
And the other person replies "asante".

I hope my comment can be useful.