I have a question, let's say I have
<input type="file" name="image" />. Will the text of the button will change if user will have different language as his system default one? If so than how can I force this button to use English by default?
The only way to change this is by replacing the button, (e.g. with SWFUpload) but I don't see why you would want to.
You shouldn't change the user's system language. They've chosen their system language for a reason, and there's an expectation that their UI will be rendered in this language that they can read/understand.