As far as foreign language study goes, it depends on your chosen subfield.
If you choose classical Greek or Roman, you will need French, German, Greek (ancient and probably modern, if you plan to research in Greece), and Latin.
Renaissance will require French, German, Italian and possibly others depending on geographic range.
20th century American will require French, German, and potentially a third language again depending on geography or specific interests.
For most of these subfields, a reading knowledge of French and German is enough. For others, a more sophisticated command is expected (for instance, Italian for Italian Ren, Greek for Classical Greek, etc.).
Since at the moment your potential research interests are broad, I'd suggest looking into terminal M.A. programs before aiming for a Ph.D. A terminal MA will give you a chance to work on your languages, narrow your focus and choose a subfield, and will also expose you to a larger pool of art historians that are available at your undergraduate institution. I'd recommend looking into the programs at Williams, Tufts, Boston University, SAIC (if you are leaning towards 20th century American), and the Courtauld.