Overall I think this proposal is fine; It's a bit unfortunate to have language dialects but on the other hand, it's more like tweaking some defaults of things which is rather understandable and e.g. scripts not having to care about most of this etc.
As I understand this proposal this is going to be opt-in, and I'd encourage the implementation to just leave it off for most swift packages (swift init created packages), so there should not be much impact on package developers. I can see the utility for small toy apps and maybe app developers.
This will also be available in a per-file basis with [PITCH] compilerSettings: a top level statement for enabling compiler flags locally in a specific file which is how I'd see it used more in scripts etc - just put it in top of a file; but the mode would need to "exist" for those settings to enable it, so I think this composes ok with the language settings pitch.