Джэк - Jehk.
Although, since Jack is short for John (Иван - Ivan) you might use one of the diminutives. But there are many, many, many, diminutives for Ivan. Vanya, Vanyusha, Vanechka, Ivanka, Ivanya, and on and on. I can recall at least 50. Russians really like nicknames.
Джим - dzhim (there is no J in Russian, so there's "D" with "ZH" as the "Z" in seizure)
Клиффорд. Pretty much a direct translation of the name.
Douwg
Matvey
Бріттні Brittni
Морин Morin
The Russian language equivalent of the name Dmitry is Dmitriy (Дмитрий).
Juana = Ivana (Ивана)
Чарльз (Charl'ze) is how you would spell the name in Russian, if that's what you mean.
Jonny, Johnny
The Russian language equivalent of the name Mark is Марк (pronounced "mark").
The Russian equivalent of the name Michelle is Мишель (Mishel).
Russian Language: РУССКИЙ ЯЗЫК.Pronunciation: Russkiy (u as in the oo in moose) yizyk (y as in the i in igloo).