Add a new lib-i18n to make message translation easier (#16942)

* Add support to better internationalization with a new lib.

* Add info about `lib-i18n` in the contributing guide.

* Use lib-i18n in M+ as well.

* Change properties files to UTF-8.

settings.gradle.kts

@@ -1,6 +1,6 @@
 include(":core")
 
-listOf("dataimage", "unpacker", "cryptoaes", "textinterceptor", "synchrony").forEach {
+listOf("dataimage", "unpacker", "cryptoaes", "textinterceptor", "synchrony", "i18n").forEach {
     include(":lib-$it")
     project(":lib-$it").projectDir = File("lib/$it")
 }