According to the Tervuren museum it is a hoax.

The document ["Charte de l'imperialisme"], which is grossly misused by some to spread unrest among ill-informed people, is false, or fake news. It belongs in the corner of conspiracy theories and is based on other fictitious documents, such as the Protocols of the Elders of Zion, a notorious anti-Semitic forgery from the early 20th century. It is, moreover, not the only fake document about the early years of colonialism to be found on the internet. For example, a similar text, entitled Discours du Roi Léopold II à l'arrivée des premiers missionnaires au Congo en 1883, has been circulating for several years.

Edit- a little further searching turns up "Leopold et les missionaires, les circulations contemporaine d'un faux" from the journal Politique Africaine. In it, Kalala Ngalamulume states (translated):

In the year 1970, an anonymous typewritten text of two to three pages circulated in Zaire (now the Democratic Republic of Congo), sometimes presented as a "Speech of King Leopold II at the arrival of the first missionaries in the Congo in 1883"....

In 1984, in the Jesuit newspaper Zaire-Afrique , the priest and historian François Bontinck demonstrated, by internal and external criticism, that this document was a forgery. Bontinck was the assumption that the text was relatively new and probably Congolese origin, even if he could draw, more or less close to the abundant critical literature in the early xx th century in Belgium, Great Britain and the United States had attacked Leopold II and his colonial project. This scientific development had no effect and the text continued to circulate, crossing the borders of the Congo, in particular thanks to the Internet network where it feeds the contemporary memories of the colony.

Let us start here from the first attested appearance of this text and its Congolese context. Its initial success must be understood in the light of the history of the conflict which opposed the Zairian State to the Catholic Church from 1971: that year, Cardinal Malula, the same one whose initiative to Africanization ("inculturation") of the Catholic Church had inspired President Mobutu, in his quest for legitimacy, the famous policy of "authenticity", had entered into open conflict with the regime by opposing the introduction of Youth cells of the popular revolution movement (JMPR) in Catholic schools and seminaries.