Collins Russian Dictionary 2nd Edition © HarperCollins Publishers 2000, 1997:
spontaneous [spɔnˈteɪnɪəs] adj (gesture) спонта́нный*спонта́нен (demonstration) стихи́йный
spontaneous combustion самовозгора́ние, самовоспламене́ние
* is used to mark translations which have irregular inflections. The Russian-English side of the dictionary gives inflectional information.
Русский: