What stopwords list is the Wolfram language using?

A little spelunking of the code for DeleteStopwords[] yields the internally used stopword list:

DeleteStopwords; (* force auto-load *)
AlphabeticSort[List @@ TextProcessing`TextModificationDump`$stopWords["English"]] // Short
   {"a", "A", "about", "above", "across", "after", "again", "against", "all", "almost",
    "alone", "along", "already", "also", "although", <<240>>,
    "within", "without", "won't", "would", "wouldn't", "yet", "you", "you'd", "you'll",
    "you're", "you've", "your", "yours", "yourself", "yourselves"}

The stop-words are documented and can be looked up as (see Details section of WordList):

WordList["Stopwords"]

As it might be subject to change so the output below can get outdated, but you can always run the function WordList["Stopwords"]:

{"0","1","2","3","4","5","6","7","8","9","a","A","about","above","across","after","again","against","all","almost","alone","along","already","also","although","always","am","among","an","and","another","any","anyone","anything","anywhere","are","aren't","around","as","at","b","B","back","be","became","because","become","becomes","been","before","behind","being","below","between","both","but","by","c","C","can","cannot","can't","could","couldn't","d","D","did","didn't","do","does","doesn't","doing","done","don't","down","during","e","E","each","either","enough","even","ever","every","everyone","everything","everywhere","f","F","few","find","first","for","four","from","full","further","g","G","get","give","go","h","H","had","hadn't","has","hasn't","have","haven't","having","he","he'd","he'll","her","here","here's","hers","herself","he's","him","himself","his","how","however","how's","i","I","i'd","if","i'll","i'm","in","interest","into","is","isn't","it","it's","its","itself","i've","j","J","k","K","keep","l","L","last","least","less","let's","m","M","made","many","may","me","might","more","most","mostly","much","must","mustn't","my","myself","n","N","never","next","no","nobody","noone","nor","not","nothing","now","nowhere","o","O","of","off","often","on","once","one","only","or","other","others","ought","our","ours","ourselves","out","over","own","p","P","part","per","perhaps","put","q","Q","r","R","rather","s","S","same","see","seem","seemed","seeming","seems","several","shan't","she","she'd","she'll","she's","should","shouldn't","show","side","since","so","some","someone","something","somewhere","still","such","t","T","take","than","that","that's","the","their","theirs","them","themselves","then","there","therefore","there's","these","they","they'd","they'll","they're","they've","this","those","though","three","through","thus","to","together","too","toward","two","u","U","under","until","up","upon","us","v","V","very","w","W","was","wasn't","we","we'd","we'll","well","we're","were","weren't","we've","what","what's","when","when's","where","where's","whether","which","while","who","whole","whom","who's","whose","why","why's","will","with","within","without","won't","would","wouldn't","x","X","y","Y","yet","you","you'd","you'll","your","you're","yours","yourself","yourselves","you've","z","Z"}

Here is a longer list using the following commands:

A = WordList["KnownWords"];
B = DeleteStopwords[A];
c = Complement[A, B]

{ a, A, about, above, across, A.D., add-in, add-on, A.E., after, again, against, a.k.a., all, all-around, all-or-nothing, all-out, almost, alone, along, al-Qur'an, already, also, although, always, a.m., A.M., Am, AM, among, an, AN, and, and/or, another, any, anyone, anything, anywhere, A-one, are, around, as, As, AS, at, At, back, back-to-back, balls-up, bang-up, be, Be, beaten-up, beat-up, because, become, beefed-up, before, behind, being, belly-up, below, between, bicycle-built-for-two, blown-up, booze-up, born-again, both, bottom-up, boxed-in, break-in, bride-to-be, broken-down, brush-off, built-in, built-up, bundled-up, burned-out, burned-over, burnt-out, bust-up, but, button-down, buttoned-down, buttoned-up, by, by-and-by, call-back, caller-out, caller-up, call-in, call-out, camp-made, can, can-do, cared-for, carry-over, cast-off, change-up, check-in, ch'i, Ch'in, chin-up, chock-full, choke-full, chucker-out, chuck-full, churned-up, climb-down, clip-on, coach-and-four, comb-out, come-on, cover-up, crack-up, cure-all, custom-made, cut-in, cut-up, D.A., dead-on, derring-do, do, DO, d.o.a., do-it-yourself, done, do-nothing, do-si-do, down, Down, down-and-out, drawn-out, dressed-up, dried-out, dried-up, drive-in, drop-off, during, each, eighty-four, eighty-one, eighty-three, eighty-two, either, end-all, enough, even, ever, every, everyone, everything, everywhere, eyes-only, face-off, factory-made, fare-thee-well, far-off, far-out, few, fifty-four, fifty-one, fifty-three, fifty-two, fill-in, find, first, F.I.S.C., flame-out, flare-up, fly-by, follow-on, follow-through, follow-up, for, force-out, fore-and-after, forget-me-not, form-only, forty-first, forty-four, forty-one, forty-three, forty-two, foul-up, four, frame-up, free-for-all, from, fucked-up, full, further, get, get-go, get-up-and-go, G.I., gill-less, give, give-and-go, give-and-take, go, go-around, go-between, going-over, good-by, good-for-nothing, goof-off, groom-to-be, half-seas-over, hand-down, handed-down, hand-me-down, hands-down, hands-off, hands-on, hanger-on, hang-up, hard-on, has-been, have, have-not, Hawai'i, he, He, head-on, heads-up, heart-whole, her, here, Here, hers, herself, higher-up, high-interest, high-up, him, himself, his, hold-down, hollow-back, hoped-for, hopped-up, how, how-do-you-do, how-d'ye-do, however, HSV-I, hundred-and-first, hushed-up, i, I, I.D., i.e., I.E.D., if, If, ill-being, in, In, IN, in-between, inside-out, interest, into, I.Q., it, IT, its, itself, I.W.W., jerk-off, Johnny-jump-up, jumped-up, keep, knock-down, knock-down-and-drag-out, knocked-out, know-all, know-how, know-it-all, ladder-back, laid-back, laid-off, lash-up, last, lay-by, lay-up, lead-in, lean-to, least, less, lie-in, lighting-up, lights-out, log-in, longed-for, looker-on, look-over, low-down, low-interest, lying-in, ma'am, machine-made, made, made-up, make-do, make-up, man-made, many, marked-up, match-up, matt-up, may, May, me, ME, mess-up, mid-May, mid-off, mid-on, might, might-have-been, mixed-up, mix-up, mock-up, more, More, most, mostly, much, must, my, myself, ne'er-do-well, never, never-never, Never-Never, new-made, next, next-to-last, ninety-four, ninety-one, ninety-three, ninety-two, no, No, no., nobody, no-go, no-one, nor, no-show, nosh-up, not, nothing, now, nowhere, odds-on, of, off, off-and-on, often, on, once, once-over, one, one-and-one, one-off, one-on-one, one-to-one, only, OR, other, our, ours, ourselves, out, out-and-out, over, own, p.a., P.A., paid-up, part, passer-by, pass-through, paste-up, pegged-down, pent-up, per, perhaps, phase-out, phone-in, pick-me-up, pick-off, pig-a-back, pin-up, piss-up, plug-in, pop-up, Post-It, press-up, pull-in, pull-off, pull-through, pull-up, pumped-up, punch-up, purpose-made, put, put-down, put-on, put-put, put-up, put-upon, rake-off, raree-show, rather, rave-up, read-out, ready-made, right-down, right-side-out, right-side-up, rip-off, roll-on, run-down, run-in, runner-up, run-on, run-through, run-up, same, Same, Sana'a, sauce-alone, save-all, sawed-off, sawn-off, say-so, schoolma'am, see, seem, seeming, see-through, self-interest, self-made, self-will, send-off, set-back, set-to, seventy-four, seventy-one, seventy-three, seventy-two, seven-up, several, shake-up, shape-up, share-out, she, shell-less, shoo-in, shoot-down, shoot-'em-up, show, show-off, shut-in, side, side-to-side, since, sit-down, sit-in, sit-up, sixty-four, sixty-one, sixty-three, sixty-two, slap-up, slip-on, slip-up, smash-up, snarl-up, so, so-and-so, sold-out, some, someone, something, somewhere, so-so, sought-after, spaced-out, spend-all, spin-off, spread-out, stand-alone, stand-down, stand-in, stand-up, start-off, step-down, step-in, step-up, stick-on, still, stock-still, stock-take, stopped-up, straight-out, stripped-down, strung-out, stuck-up, such, sum-up, sure-enough, tailor-made, take, take-in, take-up, tap-off, teach-in, than, that, the, their, theirs, them, themselves, then, there, therefore, these, they, thing-in-itself, thirty-first, thirty-four, thirty-one, thirty-something, thirty-three, thirty-two, this, those, though, three, through, throw-in, thus, tie-in, tie-on, tie-up, time-out, tip-off, tip-up, to, to-do, toe-in, together, too, top-down, top-up, toss-up, touch-and-go, touch-me-not, toward, trade-in, trade-last, trade-off, tricked-out, trip-up, trumped-up, try-on, tumble-down, tune-up, turn-on, twenty-first, twenty-four, twenty-one, twenty-three, twenty-two, two, two-by-four, two-part, uncalled-for, uncared-for, under, unheard-of, unhoped-for, unlooked-for, unthought-of, until, unwished-for, up, up-and-down, upon, upside-down, us, US, U.S.A., very, walk-in, walk-on, walk-through, walk-to, walk-up, warm-up, washed-out, washed-up, washing-up, wave-off, way-out, we, well, well-being, well-done, well-made, well-off, well-thought-of, well-to-do, what, when, where, where's, whether, which, while, whipper-in, white-out, who, WHO, whole, whom, whose, why, will, wished-for, with, within, with-it, without, work-in, worn-out, would-be, write-down, write-in, write-off, wrong-side-out, year-around, yearned-for, yet, you, you-all, your, yours, yourself }