Added language setting to allow users to change locale

[QuantFox1]

Added feature as per issue #433

[KforG]

Nice work.
Look into displaying the language name instead of the language code. Additionally how the language choice can be saved in settings.db. It would be done similarly to how the selected pool is saved I would imagine.

[KforG]

Does this compile for you? I get a mismatched types error:

backend\settings.go:198:20: undefined: langs.Getlangs
backend\settings.go:213:17: invalid operation: m.GetLang() != lang (mismatched types int and string)
backend\settings.go:214:19: cannot use lang (type string) as type int in argument to m.setIntSetting