append 'en' to application languages instead of system languages

The list of available languages for Frescobaldi is constructed from the
file names of the MO translation files; there is not a MO file for
English language, so English language code ('en') must be appended
manually to the list.
The list of system preferred languages should not be touched.
correctly translate language names in about and hyphendialog

Following feebd4d5d62e4b45936502930271899a380b2fa6, language_names does
not rely on po.setup to determine the currently used language.
As a result, on some systems and under certain conditions, languageName
returned the language names in the wrong language.
Merge pull request #182 from dliessi/system-language-detection

address oversights in recent reorganization of language detection code