No, it's not possible to change the upload button label without workarounds.
Usually, it's not necessary to localize upload button label as it is set by browser for current language automatically. Only if you'd like to use from browser setting's different language or if you want to style the upload button with css then you would use one of the workarounds you can find e.g. here or Google for "css file upload button".
Much easier it is with XPages fileDownload control. You can set the titles for "fileNameTitle", "sizeTitle" and other "...Title" properties at All Properties
tab:
![enter image description here](https://i.stack.imgur.com/B0v6C.png)
But, usually you don't have to localize titles here neither as that happens automatically for selected language in browser settings.