Most spells in Harry Potter (magic in general) are in Latin. However, at least two spells, namely Stupefy and Obliviate, are in English. Why is that? I found some sources, but they are in general talking about how magical language is Latin.
While RadarUnicorn is correct that spells such as Obliviate and Stupefy ARE pseudo-Latin just like other spells in the HP stories, there is at least one spell that seems to work with an English incantation:
Further reading (from a specialised sci-fi/fantasy site): What Wording Did Witches and Wizards in Other Cultures Use for Spells? and its many duplicates, as well as some of the answers to How could Ron possibly believe that “Sunshine, daisies, butter mellow…” is a real spell?