Склонение числительных в golang

example.go

func declOfNum(number int, titles []string) string {
	cases := []int{2, 0, 1, 1, 1, 2}
	var currentCase int
	if number % 100 > 4 && number % 100 < 20 {
		currentCase = 2
	} else if number % 10 < 5 {
		currentCase = cases[number%10]
	} else {
		currentCase = cases[5]
	}
	return titles[currentCase]
}

titles := []string{"фотография", "фотографии", "фотографий"}
number := 1
declOfNum(number, titles)

лучше использовать для number тип uint, т.к. при отрицательных значениях упадет

https://go.dev/play/p/09ZD99jsBBF

С поддержкой отрицательных чисел:
https://go.dev/play/p/RZItGDnRHYo

func declOfNum(number int, titles []string) string {
	if number < 0 {
		number *= -1
	}

	cases := []int{2, 0, 1, 1, 1, 2}
	var currentCase int
	if number%100 > 4 && number%100 < 20 {
		currentCase = 2
	} else if number%10 < 5 {
		currentCase = cases[number%10]
	} else {
		currentCase = cases[5]
	}
	return titles[currentCase]
}

func main() {
	titles := []string{"день", "дня", "дней"}

	fmt.Println(1, declOfNum(1, titles))
	fmt.Println(2, declOfNum(2, titles))
	fmt.Println(5, declOfNum(5, titles))
	fmt.Println(-1, declOfNum(-1, titles))
	fmt.Println(-2, declOfNum(-2, titles))
	fmt.Println(-5, declOfNum(-5, titles))

}