Add the JUnit tests for the Translator class

Not yet implemented.
Implement test for the Translator class
Fix bugs in Translator to make unit tests pass

- replace only first occurence of text while replacing translations in
translations.
- fix replacement at end of line.
- other characters may be stuck at the end of the key to replace (like
punctuation).
Complete and update Translator documentation
Make core methods of the Configuration class final

Fixes some Sonar warnings