Java's classic .properties files are defined to be ISO-8859-1 (Latin-1), so any character outside that range — Chinese, Japanese, accented letters or emoji — must be written as a \uXXXX Unicode escape. This is what the JDK's native2ascii command does. Paste your native text on top and the tool replaces every non-ASCII character with its lower-case four-digit \uXXXX escape, keeping ASCII, tabs and newlines untouched so the file stays diff-friendly and safe to commit.
The reverse turns a properties value full of \uXXXX escapes back into readable text, correctly re-joining the surrogate pairs used for characters beyond the Basic Multilingual Plane (such as emoji). A malformed escape is reported rather than silently ignored. Both directions run entirely in your browser, so private keys and translation strings never leave your machine.