While dispensations can be obtained which would allow a Catholic to marry a non-Catholic and for the ceremony not to occur in a Catholic Church, in this case they may be more difficult to obtain then indicated by other posters.
Commonly, when a Catholic wishes to marry a non-Catholic, they must make two promises (to which their fiancee need not agree, but must be aware of):
- That marrying a non-Catholic will not jeapordize their Catholic faith.
- That they will make every effort to raise any children in the Catholic faith.
I do not see how your fiancee could in good conscience make these promises when she is not currently practicing her Catholic faith, which could make it difficult to receive the necessary dispensations.
As mentioned, if your fiancee were formally no longer a Catholic, she would no longer be bound by Catholic canon law, and the wedding would be valid in the eyes of the Catholic Church.
Both you and your fiancee owe it to yourself to make sure that your wedding is considered valid by the Catholic Church. It will make it easier for you to deal with your in-laws, who otherwise might not consider you married. If at a later point your wife begins practicing her Catholic faith again or if you become Catholic, you would need to normalize your marriage, and the situation could also result in many guilty feelings. Most importantly, though, is that if you do not do so, you would not actually be married. (If the Catholic Church is correct.)
I would also recommend that you spend some time learning about the Catholic faith yourself before getting married. This is not to say you should convert to the Catholic faith (actually, everybody should, but that isn’t my point in writing this right now, I would equally recommend this if you were going to be marrying a lapsed Buddhist). At a later point, your fiancee may resume her Catholic faith. Any time there is a marriage between individuals of different faiths, religion becomes a possible point of contention between them, and it would be wise for you to be aware of the possible future difficulties before making a permanent commitment.