The Box Office player supports various subtitle formats as well as many languages. The default English language is very well supported as you can see below:

Pressing at the "Subtitle" menu will get you to the subtitle properties window. There you can choose among the available subtitles of the movie, change the text encoding, color, size, offset and of course disable them:

But how does the player deal with other complicated languages like ..Greek? Well, the Greek subtitles appeared on the screen although the letters had an annoying extra space between them making reading really hard:

In addition, letters using a punctuation mark won't appear at all, no matter if you have chosen the UTF-8 or any other character encoding. In any case, this is something that PatriotMemory should fix.

The good news is that DVD subtitles appear just fine:

No problems with Greek subtitles of a Blu-ray based ISO:

..although you cannot change any of the properties: