Of course the ceremony can be conducted in Latin, or other languages.
But my understanding of the official translations into English, approved for use in the U.S.A., are:
“Turn away from sin and be faithful to the gospel.” or
“Remember, man, you are dust and to dust you will return.”
(Roman Missal, Catholic Book Publishing Co., New York, 1985, page 77. This book has the above words, saying on page 78: "The blessing and giving of ashes may be done outside Mass. In this case the entire liturgy of the word should be celebrated: entrance song, opening prayer, readings and chants homily, blessing and giving of ashes, general intercessions.)
Another book approved for the USA is the Book of Blessings. The Latin edition of this book does not include a ceremony for distributing ashes on Ash Wednesday. But the USA edition does include it and has the words:
“Turn away from sin and be faithful to the gospel.” or
“Remember, you are dust and to dust you will return.”
(Book of Blessings, Liturgical Press, Minnesota, 1989, ISBN 0-8146-1875-8, page 625).
So the word “man” has been removed in the Book of Blessings. The introduction says “This blessing is an adaptation of the rite for the blessing and distribution of ashes on Ash Wednesday in The Roman Missal.” In making the adaptation they have put in a modified set of words from those approved for the Roman Missal. The Vatican have approved the Roman Missal and the Book of Blessings.
So I think it is reasonable to use one of these three, if it is being done in English in the U.S.A.:
“Turn away from sin and be faithful to the gospel.”
“Remember, man, you are dust and to dust you will return.”
“Remember, you are dust and to dust you will return.”
New translations of the Roman Missal are expected to be used in 2010 or 2011.