feat: Custom message files (#1992)

* feat: add get custom message text to admin api

* feat: read custom message texts from files

* feat: get languages in apis

* feat: get languages in apis

* feat: get languages in apis

* feat: pr feedback

* feat: docs

* feat: merge main

internal/api/grpc/text/language.go

@@ -0,0 +1,13 @@
+package text
+
+import (
+	"golang.org/x/text/language"
+)
+
+func LanguageTagsToStrings(langs []language.Tag) []string {
+	result := make([]string, len(langs))
+	for i, lang := range langs {
+		result[i] = lang.String()
+	}
+	return result
+}