flag: make the "default" word localizable

[kpym]

In flag module we can write all help messages in any language, but the "default" keyword stays in english.

It will be nice if the corresponding code could be made "translatable".

[ALTree]

Thanks for the request. AFAIK almost nothing in the standard library is localizable, english is the default. So I think it would feel a little "out of place" maybe?

[kpym]

@ALTree I understand, but this library is an user interface library. Heaving some log message in english is ok, but producing user interface that has only few non translated words is strange.

Probably this should not be properly "translatable", but only made customizable. As we have flag.Usage maybe we can have flag.DefaultMsg that can be defined to display the default values ?

[ianlancetaylor]

It seems to me that programs that support localization should use the flag.Usage function to control the usage message. They shouldn't call PrintDefaults but should use their own locale-appropriate mechanisms.

[ianlancetaylor]

Alternatively, use an io.Writer that reads the documented format of PrintDefaults and applies its own localization as appropriate.

[kpym]

@ianlancetaylor Thank you for your advice. I think you are right.

[kpym]

Here is an example of how we can do this:

// Replace "default" with "par défaut" in `flag.PrintDefaults`
func PrintDefaultsFr() {
        // save and restore the output (os.Stderr by default)
	original := flag.CommandLine.Output()
	defer flag.CommandLine.SetOutput(original)
	
	// redirect the output to new buffer
	buf := new(bytes.Buffer)
	flag.CommandLine.SetOutput(buf)
	
	// print the defaults in french
	flag.PrintDefaults()
	original.Write(bytes.ReplaceAll(buf.Bytes(), []byte("default"), []byte("par défaut")))
}

Check it in the playground.