Entity Reference on multilingual sites
While this doesn't answer the behavior of the Entity Reference fields over different languages, switching the entity selection mode to a view seems to have solved it.

As suggested by Lester, I'm still unsure if the default 'Simple' selection mode was taking issue with the title of my fields, although rather than using the Title module - which can trip up other modules, i have found - I tend to keep the old title field and label it 'Administrative title' while adding a separate 'Translatable title' to work with - so in that sense there shouldn't have been an issue with the 'Simple' selection method.

Either way, setting up an Entity Reference View which displays and searches the 'Translatable title' of my Features node seems to have worked!



EDIT:

@Dan - these labels, are they node titles or fields which you created?

The issue with standard node titles is that they are different from other fields and as such are not 'entities' - this means that entity translation will not work on them. The workaround is to either use the Title module to replace the default title field with an entity (I had some issues with this), or use a different field which you create yourself.

Once the field you need to translate is an 'entity' (basically an field you create yourself, or one generated by the Title module) you can enable translation for it when adding the field to a content type.

Next you need to create a new view containing whatever content you want to use and click the + button to add an 'entity reference view'. After configuring this view with the fields you want to display and saving you should find that when editing the Entity Reference field in your content type, you can change the Entity selection mode to Views and select your newly created view.

I hope that makes sense!