I am afraid, this is not possible at the moment. There is no built-in diacritics-insensitive search in those functions.
Though, a few options would be:
Additionally store a non-diacritics-version of the search-word in the document, so that you can query it that way.
Write a custom function in which you would implement that kind of functionality. Though, I suspect that this would have a negative impact on performance. See the docs on that: https://www.arangodb.org/manuals/current/ExtendingAql.html
Open a feature-request-issue to implement this kind of functionality in ArangoDB. Who knows, maybe it gets implemented? (https://github.com/triAGENS/ArangoDB/issues)