In Spanish, the composer's surname is Beethoven.
In case of an adaptation to Spanish spelling, it would be written Beetoven or Betoven, because the TH is inexistent (EE exists: reedición, e.g.).
If you google "Bethoven", from 43.600 websites, only 5.250 are in Spanish and 17.500 are in English
Lumia
http://onomastica.mailcatala.com