"Mám v mléku mouchu."

Translation: I have a fly in my milk.

[gSee4]

Why is v mléku but not v mléce

[Praha2017]

Both are correct, I think.

[ion1122]

The word "mléčný" means "dairy", not "milk". Milk is a dairy product, but dairy also includes cheese, butter, etc.

[BoneheadBass]

According to this link, both are valid for locative singular (and there are also two choices for locative plural).

https://en.wiktionary.org/wiki/ml%C3%A9ko#Declension

[Siarhei567254]

Why do we need to use "my milk"? In Czech version there's no "v MÉM mléku" or something like that...

[VladaFu]

You don't, "the milk" is also accepted. But it is customary to use the possessive in English here, English uses possessives much more than Czech does.