.

Similarly, what do you say when you say so?

Some of my favorite ways of sarcastically saying "if you say so" are:

1. "Oh, I see you believe it to be so, so now it must be so."
2. "Looks like someone's making the best of their kindergarten education."
3. "Way to go Einstein.
4. "Well that's clear as mud."
5. "Brilliant as a lump of coal."

what do you say meaning in English? (idiomatic) Used to ask someone if they are willing to do something.

Likewise, people ask, why didn't you say so meaning?

To add on to the others: "Why didn't you say so" is more of a general idea while "why didn't you say that" is more specific to the words. Another example is: Said is what he said while "was like" is the idea/ emotion behind it.

Is that so meaning?

I see "Is that so" as confrontational, typically expressing disbelief or challenge rather than merely doubt. "Oh, really?" sometimes expresses doubt, and sometimes indifference. The specific meaning of either phrase strongly depends on context and manner.