We have this saying in my country "if grandma had a mustache she'd be a grandpa".

I like my country's version better: "if grandma had wheels she'd be a bicycle".

Which language/country is this?

I will use this in English regardless. I like it.

Mexican Spanish. I don't know if they also use that saying in other Spanish speaking countries besides Mexico. In Spanish it goes: "Si mi abuelita tuviera ruedas, sería bicicleta".

If your aunt had nuts she'd be your uncle

That has the same basic idea as the mustache version, but is pithier. I still prefer the surrealism of the bicycle.

You'd probably do better on HN to substitute 'decrying' for 'bitching'.

Noted, thanks. Didn't realize that was the issue.