added i18n in golang

content/Golang/i18n with golang and text package.md

@@ -0,0 +1,92 @@
+There are no good examples of [golang.org/x/text](https://golang.org/x/text)  uses and description of how pluralization made. So, this is it.
+
+#### Generating translations
+
+> You should specify valid [ISO-3166 locale code](https://www.iso.org/obp/ui/#iso:code:3166) to get plurals working!
+
+Write your code:
+
+```go
+import (
+	"golang.org/x/text/language"
+	"golang.org/x/text/message"
+)
+
+lang := language.MustParse('ru-RU')
+printer := message.NewPrinter(lang)
+
+count := 1
+printer.Sprintf("%d butterflies", count)
+```
+
+Then run generator for your locale (mine is `ru-RU`):
+
+```shell
+gotext -srclang=en-GB update -lang=ru-RU \
+	-out=translations.go github.com/path/to-output-folder
+```
+
+This will generate `out.gotext.json` in `/path/to-output-folder/ru-RU` of your project. Copy it to `messages.out.json`.
+
+#### Translating and adding plural forms
+
+Change `"translation"` value in generated `messages.gotext.json` to `string` or `object` of the form shown below if you want pluralize strings, ==then run gotext command again==, it will generate `.go files with translations`.
+
+According to [source files of gen_common](https://github.com/golang/text/blob/master/feature/plural/gen_common.go#L19), there's a couple of forms: 
+- `=N`, `!=N`,  `%N`  for exact matches
+- `zero` | `one` | `two` | `few` | `many` | `other`  - they're different for each language, so, 
+
+```json
+{
+  "id": "{Count} butterflies",
+  "message": "{Count} butterflies",
+  "translation": {
+	"select": {
+	  "feature": "plural",
+	  "arg": "Count",
+	  "cases": {
+		"one": {
+		  "msg": "{Count} бабочка"
+		},
+		"few": {
+		  "msg": "{Count} бабочки"
+		},
+		"many": {
+		  "msg": "{Count} бабочек"
+		},
+		"other": {
+		  "msg": "{Count} бабочуль"
+		}
+	  }
+	}
+  },
+  "placeholders": [
+	{
+	  "id": "Count",
+	  "string": "%[1]d",
+	  "type": "int",
+	  "underlyingType": "int",
+	  "argNum": 1,
+	  "expr": "count"
+	}
+  ]
+},
+```
+
+#### Using go-generate
+
+Write this at some `.go` file:
+
+```go
+//go:generate gotext -srclang=en-GB update -out=translations.go -lang=ru-RU github.com/muerwre/vault-golang/internal/api
+```
+
+Then run go generation:
+
+```shell
+go generate ./...
+```
+
+#### Useful links
+- [All available forms in this .go file](https://github.com/golang/text/blob/master/feature/plural/gen_common.go#L19)
+- [Answer, that solved it for me](https://community.crowdin.com/t/plurals-in-gotext-json-files/600)