Exegesis:

ophthē ‘was seen’: this passive form (from horaō ‘see’) is commonly used with the meaning ‘appeared,’ of beings who make their appearance in a supernatural manner (cf. Lk. 1.11; 22.43; 24.34; Acts 2.3; 7.2 etc.).

sullalountes (only here in Mark) ‘talking with,’ ‘conversing with.’

Translation:

Elijah with Moses must be rendered in some languages as ‘Elijah and Moses.’

The transliteration of Old Testament proper names should correspond to the form which they have (or will have) in any Old Testament translation.

If a direct form of expression is required, appeared to them may be rendered either ‘came into their view’ or ‘they saw.’

In some languages (e.g. Zapotec) special grammatical forms are required in speaking of persons who have died. Accordingly, Elijah and Moses must be so designated, regardless of the account of Elijah’s miraculous ascent to heaven. In other languages all such patriarchs must be addressed or spoken of with honorific language.

Quoted with permission from Bratcher, Robert G. and Nida, Eugene A. A Handbook on the Gospel of Mark. (UBS Handbook Series). New York: UBS, 1961. For this and other handbooks for translators see here .