How do I convert date to the string in go using locale?

I convert date to a string this way:

d.Format("Mon 02. Jan")

and I get something like

Fri 27. Jan

How can I switch the locale and get the string in other language?

You can't. The Go standard library does not contain localized month, day and zone names. The names are wired into the time package.

For example, the name of the months returned by Month.String() are stored in the unexported time.month global variable:

var months = [...]string{
	"January",
	"February",
	"March",
	"April",
	"May",
	"June",
	"July",
	"August",
	"September",
	"October",
	"November",
	"December",
}

func (m Month) String() string { return months[m-1] }

Similarly, names of weekdays come from Weekday.String(), stored in unexported variable time.days.

Having said that, there may be 3rd party libraries supporting your needs. Here's an incomplete one which might be of some help to you: https://github.com/mattbaird/go-i18n-formats

As shared by Igor Chubin below, this 3rd party lib is much more complete: https://github.com/klauspost/lctime

Also note that while providing a general, multilingual time formatting package is not an easy task, if you really need it, you could take the time package, copy it to your project and just translate the names to the language you need.

Also note that supporting a low number of languages and low number of layouts, it's easy to create the formatting yourself.

For example, the code below formats a given time.Time value in Hungarian, using the layout you used in your question:

func Format(t time.Time) string {
	return fmt.Sprintf("%s %02d. %s",
		days[t.Weekday()][:3], t.Day(), months[t.Month()-1][:3],
	)
}

var days = [...]string{
	"Vasárnap", "Hétfő", "Kedd", "Szerda", "Csütörtök", "Péntek", "Szombat"}

var months = [...]string{
	"Január", "Február", "Március", "Április", "Május", "Június",
	"Július", "Augusztus", "Szeptember", "Október", "November", "December",
}

Testing it:

fmt.Println(Format(time.Now()))

Output on the Go Playground:

Ked 10. Nov

Output on my local machine:

Pén 27. Jan

You can use a replacer as quickfix for the problem.
Example in german:

r := strings.NewReplacer(
            "January", "Januar",
			"February", "Februar",
			"March", "März",
			"April", "April",
			"May", "Mai",
			"June", "Juni",
			"July", "Juli",
			"August", "August",
			"September", "September",
			"October", "Oktober",
			"November", "November",
			"December", "Dezember", )

	
time := inputTime.Format("2 January 2006")

outputString := r.Replace(time)