I want to include a philosopher's quote in an essay (APA). Is it okay to do so? If yes, how should this be formatted? Should the quote be in italics followed by an English translation in square brackets [ ] ?
Reasons why you might want to include the original:
There are issues related to the precise wording and terms used in the original, which you pick up and elaborate on.
There is no reliable translation available, i.e. no published article contains the quote you would like to refer to. In this case, I would translate the quote myself and add a footnote along the lines of
"original text" [source] (translated by this article's author).
Understanding the language the original is in is expected from the reader, e.g. in an article about the particular language. No need to include the translation in that case.
In any other case, I would stick to the translated version solely (and attribute it to its source as usual).
If memory serves, then I have not read a quote yet which was not in the original language with a translation appended at the end.
If you want to include a quotation (even if it is just to make the text visually more appealing) then you should use the original formulation with a translation appended at the end.
In addition to my comment above, I want to mention that, if you will decide to go ahead and include both the original quote and translation, them, as far as I know, at least per this APA Style blog post and comments, you should not only, as you said, place English translation in square brackets after the original, but also present the original quote italicized (as you mentioned) and transliterated into the Latin alphabet, if the original language's alphabet is different from Latin.