For the first issue, do you have a concrete use case?

The translation strings are indeed quite simplistic, as they are often written by non-engineers unfamiliar with concepts like escaping. One approach is to replace the text to be escaped itself with placeholders, for instance:

printf("Single-number set: { %d }", num)

could rewrite to something like

"Single-number set: {OPEN_BRACE} {Num} {CLOSE_BRACE}"
With the placeholder Num as before and OPEN_BRACE="{" and CLOSE_BRACE="}"

Another approach would be to, like go templates, to allow for user-specifiable open and close markers.

What is best depends a bit on what users wants and needs, so input is welcome.

The particular use case I hit was in writing an error message to be generated by a JSON parser when it expected to see an object. The message in English was expected '{' or 'null'.

You are correct that this could be resolved by treating the open brace character as something to be interpolated into the message. That seems cumbersome from a developer perspective, but I take your point that it might be more intuitive for a translator.