This is determined at the browser level, if you have not moved to something like flash-based uploader (or <iframe> trickery), you cannot configure it. Whatever language or localization the user browser uses (usually based on the OS settings), this language will be displayed on the "Browse ..." button.
If your page is intended for consumers, probably already in that language, then you are all set, the default behavior works. If people with the settings for another language come, well ... what language they chose, so it should be an intuitive button label, even if it doesn't match.
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