Unfortunately due to China restrictions there is not much anyone on firefox End user forum can do to make that change. You might want to contact payonner support and ask them about the problem as well.

Some sites automatically redirect you to a site in the local language, based on where your IP locates you. There may not be much that you can do about this unless the website offers a link to an international version or to a version in your preferred language. Such a choice is stored in a cookie, so you need an allow exception to keep such a cookie. There is usually a link on such websites choose a language or go to an international version that allows to choose the language.

A possibility is to use a VPN service that locates you in this specific country.