Belén: This is the Spanish name for Bethlehem. It isn't unusual for names of cities, particularly those well-known centuries ago, to have different names in different languages. Interestingly, in Spanish the word belén (not capitalized) has come to refer to a nativity scene or a crib. It also has a colloquial use referring to confusion or a confusing problem.
Cuán: Use of cuán instead of qué to mean "how" is infrequent in everyday speech and is limited mostly to poetic use.
Tú estás: Referring to a town as a person is an example of personification.
Mas: Without the accent, mas is basically synonymous with pero, meaning "but." It is not used much these days outside of literature and poetry.
el Mesías: The Messiah.
Nacido el Mesías ha: In usual speech, this sentence would be ordered as el Mesías ha nacido. The word order has been changed to fit the rhythm of the song. There would be no other reason word a sentence this way.
Derredor: You won't find this word listed except in larger dictionaries. In this context, it is probably best translated as "surroundings."
Alábenlo: This is a use of the verb alabar ("to praise") in the plural subjunctive. The fact it is in a plural gives a clue that the subject of the verb is the following plural noun, astros. Alábenlo los astros means "may the stars praise him."
Nuevas: Although noticias is the more common word for "news," nuevas can also be used.
Proclamad: If you're familiar with only Latin American Spanish, you might not know this verb form well. The -ad is the command that goes with vosotros, so las nuevas proclamad means "you (plural) proclaim the news" or simply "proclaim the news."
Aún: This adverb typically means "still" or "yet."
Aún lo puede recibir el manso corazón: This is another example of inverted sentence order. This sentence can be translated as "the quiet heart can still receive him." The Spanish here is a paraphrase of the original English "meek souls will receive Him still."
