Mention string templates should not be used as they cannot be localised

[aral]

Discussed in #146

^{Originally posted by aral November 5, 2021}
Reference: https://github.com/elementary/docs/blob/master/writing-apps/code-style.md#string-formatting

It would be good to provide a reason for why string templates are discouraged, given their ergonomic advantages over remembering every % code on the planet.

I haven’t been able to find any information on how translations work/don’t work with string templates but I’m assuming they are complicated at the very least. Is this the reason why they’re discouraged?

I’ve now confirmed that string templates are not picked up/included in .po files.

We should mention this reason in the docs.