The MHRA Style Guide Online
A Handbook for Authors and Editors • Third Edition
From Chapter 2, 'Spelling and Usage'
There is great inconsistency between dictionaries (and sometimes within the same dictionary) as to the use of accents and other diacritics in words borrowed from other languages.
Two cases are, however, clear:
(a) When a word or, more often, an expression is still felt to be foreign (and an objective decision is not always possible), all diacritics should be retained, e.g.:
aide-mémoire, ancien régime, à la mode, Aufklärung, la belle époque, bête noire, cause célèbre, déjà vu, éminence grise, Führer, lycée, maître d’hôtel, papier mâché, pièce de résistance, più, raison d’être, señor, succès de scandale, tête-à-tête
Such words and expressions are often italicized (see also 7.2).
(b) Words ending in -é retain their accent:
blasé, café, cliché, communiqué, exposé, fiancé (also fiancée)
In such words, any other accents are also retained, e.g.:
émigré, pâté, protégé, résumé
We recommend that, except as provided for in (b) above, diacritics should be dropped in the case of words that have passed into regular English usage, e.g.:
chateau, cortege, creche, crepe, debacle, debris, decor, denouement, detente, echelon, elite, fete, hotel, matinee, naive, precis, premiere, regime, role, seance, soiree
For the use of accents on capitals, see 6.6.