[TECHNICAL SUPPORT] LPS-35920 Restrict LanguageServlet file download function to work only for valid languageId and valid language key pairs

Hi Tamás,

There's a possibility to prepare links to download text as plain text file which seems that the portal hosts that file with that possibly offending or obscene content.

In this PR I limited the resource serving to valid languageId and language key pairs so this servlet will do only it's expected job.