The writings.. are mostly in the form of letters to believers and inquirers. A great many of His talks and addresses have also been recorded.. (and) written descriptions (by pilgrims).. (66:5)

His teachings are thus very completely preserved, and they cover a very wide range of subjects.. giving more detailed applications of the general principles laid down by Baha'u'llah. (67:1)

He spoke in Persian, Arabic and Turkish. In His Western tours His talks and addresses were always interpreted. (67:2)

End of Quote

Baha'u'llah & the New Era
J. Esslemont