What language is your Paypal account in? The language of your Paypal account decides what language you get in Paypal.
Wenlan is right. Here are some more details:
1. The form language has nothing to do with the UI language for the PayPal page. The language used for the PayPal page will depend on the language settings in your PayPal account.
2. There 2 things related to locale in the PayPal page your user will see: the default country for credit card payment and the UI language used to display the page.
3. The default country for the credit card payment will always the same as the country of your PayPal business account registered to your form. That is, if your PayPal account was created in PayPal US, the account country will be US, and the default country shown to your user for credit card payment will also be US.
4. When you log in to your PayPal account, you are able to choose your UI language (usually at the upper right corner. Please see screenshot below). Not all languages are supported for PayPal accounts in different countries. For example, if your PayPal account is an US account, you can choose English, Spanish, French and Chinese, but if your account is an Japanese account, you only have choices of English and Japanese.
Therefore, if your customers are mainly Japanese, you'd better to create a Japanese PayPal account, make sure you choose Japanese for your UI, and register it to your form, so that when your customers check out your form, they will see Japanese payment page.
Please let me know if you have more questions.
Senior Computer Scientist
Adobe Systems Incorporated