Languages make everything so complicated! I just discovered the autocomplete bit on my reference fields wasn't working - until I tried searching by another language, and then the results gave me a surprise:
My homepage, originally in French, has had an English translation added, but whilst editing in English, I had to search with the French title in order for the page to come up... with the English title! Here's how to solve this...
My site uses Entity translation and the Title module (which I usually recommend as the best way of making your site translatable), and this field is an Entity Reference field. The field is configured to use the 'simple' selection method, which is the default.
Instead, I needed to use Views to build the list of content that can be referenced, to be used instead in that 'Mode' setting. Create a view, add an 'Entity Reference' display, and add the 'Field: title' field (use the search box to make this easier), instead of the 'Content: title' field that gets added normally. The following screenshots should help show how to do this:
This 'Field: title' field is the properly translatable field that the Title module sets up, whereas 'Content: title' is the original title that is not truly translatable. I also suggest adding the filter for 'title_field:language', filtering to the current user's language -- otherwise the autocomplete search will search across all languages! (So in my example, I could search for either "Homepage" or "Page d'accueil" to find the page.) By the way, if you want reference taxonomy terms, instead of using 'Field: title', you'll want 'Field: name' (and the name_field:language filter) instead. Some other entity types use other wording too.
Next, configure the settings in the format section of the admin screen to use the new field for searching. Finally, once you've saved the view, edit the reference field and change that mode setting from using the simple method to instead use views, and pick the view you just created.
This now works, I can search for 'Homepage' and get 'Homepage' as a suggestion in the autocomplete, as I had originally expected! It's bizarre, but hopefully this discovery and solution might help someone else :-)
This also demonstrates how you can use views to search for other fields on content, not just their titles - this isn't just for translatable sites! You can even use a different combination of fields in the view to change what's displayed in the suggestion box (e.g. adding product codes, images, etc).