Allow language selection (#191)

* Add dropdown for language

* Use vue-i18n to translate strings

* Tweak header styling

* Add Chinese languages in

* Typo

* Get language from browser (#193)

* adaptive system language

* Modify the adaptive system language

* Remove dead code

* Delete lang default values

* Move browser language detection to util

* Remove todos

* Fix global PHP dropdown

Co-authored-by: 墨娘 <61287199+moniang@users.noreply.github.com>