Replaces the wording of one string for one language.

• The fields go inside a string object. The string is identified by the id in the URL.
• language and branding are read from the body, not the query string. Leaving out
  language writes the default (base) wording, not a translation.
• Sending the wording the string already inherits removes your override instead of storing
  it, so the string goes back to the inherited wording.
• Saving the first string in a language adds that language. There is no separate step to
  add a language.

The change applies to checkouts opened after it is saved. A checkout session that was already
created keeps the wording it was created with until it expires.