This word has that sort of foreignness to it that one gets from words not of a distant place but of a distant time. The schal in particular may strike a few among us (e.g., me) as also being a Middle English spelling of shall. But this word does not relate to shall; rather, it comes from Old Teutonic skalko, servant. The sene may make you think of the Latin root for “old”, and you’d be right about the “old” but it’s again by way of Old Teutonic (which has a common origin with Latin, way back). But more amusingly, this word, which sources from Old Teutonic and resembles a Middle English spelling of an original English word, and has the sch that normally shows up in words from Italian (and Latin), Greek, Dutch, and German, came to us most directly from French – Old French took the Teutonic seniscalc and made it into seneschal, which form was borrowed directly into English at a time not long after French was the ruling standard, right around 1400. So it was a Middle English word, but its form comes from the French. Find me another French loan with sch!
But, now, how shall we say it, and how shall we use it? Because it came from the French, the sch is the sound we normally spell sh. But we still stress the first syllable. As to its object, a seneschal is a majordomo of a sovereign or great noble, or a cathedral official, or in some cases (as in the Channel Islands and the Society for Creative Anachronism) an administrative or judicial officer. Does that make sense, or will you challenge it? You may find your lens aches at too much exposure to this word, with its medieval orthography in which may be found chess, lances, lashes, chases, and other such scenes (but at least you may heal sans leech). But no need to get all mixed up. This word is not like a spiced olive that you may drop into the martini of any conversation; rather, it is a ball of incense for a period piece book. Readers of medieval murder mysteries will surely see it soonest.