 To change the language of the QGIS interface, go to settings, options, and there, under general, you check the box override system locale. There you can choose, for example, American English, and for the format of numbers, dates and currency, you can choose whatever you want, I'm going to choose here English United Kingdom, and you see here it shows an example of how it will look like. For these settings you need to restart QGIS, so if you click OK, then the new settings are applied when you have restarted QGIS.