When you use our hosted signup page, you can submit an english page by adding the following URL suffix /portal/signup.html#/"Planvariants-Id"?language=en
If you use our hosted order portal, you can easily display an English translation of the order page with the URL suffix /portal/signup.html#/"Plan Variant ID"?language=en for your customers. You can also access a French order page with "?language=fr". You can translate the titles and headings of the respective languages in "Settings > Integration > Self-Service > Hosted Signup". These translations can also be output for other languages.
You can also prefill the country of the billing address with a URL query string. Several parameters are separated by"&" https://de.wikipedia.org/wiki/Query_String You can find a list of fields that can be pre-filled here: https://developer.billwerk.io/Docs/GettingStarted under the section "Hosted Customer Self-Service".
The two-digit ISO 3166-1 code is used for the country codes. You can find a list here: https://en.wikipedia.org/wiki/ISO_3166-1_alpha-2 The code for the language (?language=) should be written in lower case, the code for the country capitalized. For regional languages use "-" and the country code.