Translation:How does Spanish relate to French?
Did you mean you tried "How spanish relates to french"? That's not correct in English.
(if that wasn't the question, please ignore; also, this part is about English, not Esperanto) To make questions like that in English, you need to turn the main verb "Spanish relates to French" into a helper-verb pair "Spanish does relate to French", then move the helper verb to the front "Does Spanish relate to French" to make a yes-no question, or to after the question word "How does Spanish relate to French"
Similary, your question should be: 'Why is "relates" wrong'; "is" is a helper verb, so it gets moved by itself.