How do you say ESTAR DE LUTO in English?

The correct term is TO BE IN MOURNING.
It is by definition that period during which the relatives and friends of someone who has recently deceased grieve. The ways in which this expression of sorrow is manifested varies from culture to culture. If we take the Occidentals as an example, we will find a preference for dressing in black or at least wearing something of that color as the ribbon in the picture suggests. Even among Occidentals there are differences some subtle, some quite evident, concerning the way mourning is performed, so to speak. In the U.S. for example it is customary to dress in black when attending a funeral but wearing black after that is not really necessary. In Ecuador, what society dictates on this aspect has quite relaxed and is no longer necessary to wear black all the time as long as a proper and respectful behavior is observed. In other countries such as India where Death is not believed to be the end but a passage to the next stage in the quest for enlightenment, extended mourning is not advised because it could hinder the passing of one soul from this level of existence to the next. In the past and still in the present, the members of some royal families wear white not black when they are mourning.