To whom it may concern in spanish?

It is a single Word 'whomsoever', not two Words 'whom' and 'soever' as it is mentioned by you. The sentence 'To whomsoever it may concern' means the matter which would follow this sentence may have something with an unknown person who may have an interest in it.

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Better still: To whom it may concern:

By a well established convention this is a common heading for a testimonial. Unlike a reference, which is confidential, a testimonial is given to a job applicant, who can then pass it on to an employer, college, etc.