i18n: suppress duplicated languages

The list of translations from Transifex contains "en", and we
add it explicitly a second time -- instead of fixing those and
needing to ensure that the source language is returned, or optionally
adding "en" if it isn't in the list already, just de-duplicate.

FIXES #2257

CMakeLists.txt

@@ -513,6 +513,7 @@ if(NOT CALAMARES_RELEASE_MODE)
     list(APPEND CALAMARES_TRANSLATION_LANGUAGES ${_tx_incomplete})
 endif()
 list(SORT CALAMARES_TRANSLATION_LANGUAGES)
+list(REMOVE_DUPLICATES CALAMARES_TRANSLATION_LANGUAGES)
 
 add_subdirectory(lang) # i18n tools