A Spanish document from Spain uses 'castellano' rather than 'español' to describe the language. For a U.S. audience, the translator should:
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A
Translate it as 'Castilian' to preserve the distinction
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B
Always render it as 'Spanish' since the distinction is irrelevant in the U.S.
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C
Use 'Spanish/Castilian' with a note explaining the regional preference
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D
Replace it with 'Iberian Spanish' for clarity