i18n: Fix system language detection for generic English & improve dialog box and detail text when using system language

[SunderB]

This PR fixes a few issues when using the system language:

• Fix system language detection when the system language is generic English (i.e. has the lang code of "en") - should hopefully fix system language on (OSX, Raspberry Pi) is not properly detected #3215 and fix Incorrect language #3290
• Only show the language that will be loaded in the confirmation dialog when switching to the system language
• Only show the language that is currently loaded in the text below the language setting when using the system language
• Also, add native language name for Arabic

[samaaron]

Brilliant stuff, thanks @SunderB