[BUGFIX] only register (Sub)PageLanguageOverlayProvider if pagesLanguageConfigurationOverlay is activated

[franzkugelmann]

If setting pagesLanguageConfigurationOverlay = 0 then in this project all fluidpages configuration is done only in the default language. Thus the (Sub)PageLanguageOverlayProvider is not needed at all and in fact causes an error

Call to a member function getOption() on null

if the page_language_overlay record is edited.

I am not 100% sure if this assumption is correct, but solves the problem in my instance without creating other issues, as far as i can see.

[NamelessCoder]

I did a little change to make sure the providers only load when the option is explicitly enabled. Honestly, I thought these providers were already excluded from loading when the option was off - it definitely isn't the intention to have those running if you disable that option!