Consider format-<lang> and format-<lang>-<variant>

[ritze]

What does this PR do?
Like for hyprland/languages and niri/languages sway/languages can be configured for alternative languages and variant names:

Example:

"sway/language": {
	"format": "Lang: {}",
	"format-en": "AMERICA, HELL YEAH!",
	"format-tr": "As bayrakları"
}

This only works if format uses {}. I guess more work is required here, since all */languages plugins have different behavior. For the first approach I decided for the easiest solution. Maybe we should clarification the expected behavior and update the PR.

I also think that the code of all */languages modules can be merged to a common code base and only the glue code for each desktop environment can be separate.

Related issues
unknown

Checklist

• Code is formatted with clang-format
• Builds locally (ninja -C build)
• Man page updated for any new/changed user-facing option (man/)
• Tested against the affected module(s)