Hello Ciaran,
I created a multi-lingual site and had to enable the hornav-position to see the mainmenu in every language.
Now hovering over the menu-items results in no background color and no visible font for the menu-items.
When using no hornav-position everything hovers fine but then I only see one mainmenu ( in dutch ).
Site = joomla 3.5.1 - http://lukthcl31.thirtyone.axc.nl/hik/nl/