Collins Russian Dictionary 2nd Edition © HarperCollins Publishers 2000, 1997:
soften [ˈsɔfn] vt смягча́ть (смягчи́ть perf)
vi смягча́ться (смягчи́ться perf)
* is used to mark translations which have irregular inflections. The Russian-English side of the dictionary gives inflectional information.
Русский: