Support preferred language

[ufl-taeber]

Add a preferred language property to the Client, so that, for example, when there are multiple language strings in a call to Single(), the one with the preferred language will be returned.

  Results() => ["color"@en-US, "colour"@en-GB, "couleur"@fr-FR"]

By default, Single returns first item.
  Single() => "color"
  
If the preferred language is set and a string with that language suffix exists, it is returned instead.
  client(..., "fr-FR")
  Single() => "couleur"