Fixed filename issue and russian translations in tests

There was a problem when generating the target filename containing the newly generated translations. For example, If I had a filename like: ````messages.en.yml````, when I wanted to translate it to "es", it just replaced any "en" matches:

    mensagen.en.yml
    mrusagru.ru.yml
    mfrsagfr.fr.yml

Fixed replacing "str_replace" with a regular expression. Also, I did some changes in the tests because it seems the Russian translation has changed since the last update.