The original text of Matthäus Evangelium is Septuagint Greek. It's likely that the original too was a translation from a now lost source.
Interlinear glosses are supplied by biblehub. https://biblehub.com/interlinear/matthew/26-41.htm
The de Wette translation up to ἵνα is trying to capture Greek grammar. He is clearly a Dutchman in name (born in Weimar, it's notable that Erfurt was Hansestadt in the middle ages) and he may have taken a rather methodical approach to the translation (“Seinen Lebensunterhalt verdiente er in Jena unter anderem durch Übersetzungen und durch die Mitarbeit an Schillers Journal.” de.WP)
grēgoreite kai proseukhesthe
Sie bedurften des Wachens für sich selbst, des leiblichen und des geistlichen, des Gebetes.
The Greek verbs are active present imperative and (middle / passive) present imperative.
lsj.gr yields results for both verbs: gregoreo a late form of ἐγείρω, proseuchomai a compound (πρός, εὔχομαι [Bailly abrégé]); the details are immensely confusing and it looks a lot like de Wette simply resorted to make stuff up.
Sie bedurften
This captures both 2nd person (honorific Sie) and a sense of imperative, not unlike English "you ought to", "you need".
That's an interesting problem because dürfen is apparently derived from a subjunctive (cf. DWDS/Pfeifer). Subjunctive is a polite circumscription of the imperative. German Konjunktiv, which seems to cover this usage (frequently würden sie "would you", "do you mind", Seien Sie or Sind Sie mal so nett "could you", "do me a favor"), is in most cases not morphologically distinct from indicative, often as preterite (Ich wollt ich wär ein Huhn).
This turn of words today appears an entirely stilted mangling of semantics. It's incomprehensible to me.
des Wachens
Transitive bedürfen commands the genitive case. The inflection of the article is remarkable because similar verbs command accusative (ambiguous, ich brauche das, archaising ich gebrauche des [e.g. Goethe]).
Following LSJ, “to be or become fully awake, watch” are possible translations to Gr. gregoreo. Nominalized Wachen from infinitive wachen is unremarkable and the meaning may include watch (as of Nachtwache "night watch", wachen "to guard"), but it feels too speculative to conclude anything from it at this point.
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Why is »des Wachens« declined in the genitive case (or is it?)?
Seems to be entirely conventional. Cf. Grimm: “der unterschied des partitiven gen. und des acc. ist wie sonst, ich bedarf des weines, etwas von dem wein, ich bedarf den wein, habe ihn überhaupt nöthig. oft aber tauschen beide casus gleichgültig.” (DWDS/DWB, empasis mine: the case is variable). See there also with following dasz or damit, with infinitive, with reflexive sich bedürfen.
It may be notable that Ancient Greek ἵνα frequently appears with τόσ in command of the genitive, too. In fact, conj. dass is a plausible translation of ἵνα, and this is not well explained. We have similarly genitive wegen des, conj. indes, unterdessen, und es etc. See e.g. all in one phrase, including subjunctive: DWB bedürfen 7)
“doch also, dasz [.CONJ] jederzeit das eine theil davon zimlich lang für die wunden heraus lange [.SBJV], damit [.DEM-ADV], wann es sich bedarf [.COND-INDF-REFL-PTCP], du es leichtlich könnest widerumb heraus ziehen. [.INF] Würtz 346;
https://www.dwds.de/wb/dwb/bed%C3%BCrfen
The glosses are provisional. Case is indeed ambiguous in this instance.
It may be helpful to assume that accusative casus was not available to de Wetten, leaving partitive genitive as the only option, assuming that Sie is an obscured accusative case, which be amended with an appropriate w-word (compare wan [Fischart, op.cit.], English he-who, Latin quam, quisquam etc.) since “ihr” in the citation is telling of a notable contras.
für sich selbst
The text does not warrant this adding. One might think of nimm dich in Acht "beware", pass auf dich auf “take care" and similar phrases. This isn't a common thing to say, # wachen Sie für sich selbst "watch yourself", not to mention substantive Selbst (contra possessive sich). A distributive "each other" is possible.
However, it is much more likely in my humble opinion that it tries to capture the mediopassive of the following verb, perhaps as a matter of confusion (cf. Dutch zich also “something indicates an unintended result with many (also intransitive) verbs”
des leiblichen und des geistlichen
This is a paranthetical and highly ambiguous. As paranthetical, it connects the two verbs to give them contrastive meaning. Clearly inflected as adverbs, leiblich and geistlichen belong to the respective verbs closest in context. Alternatively to be read as apposition, it implies that the second verb is part of an enumeration which specifies the first verb, to resolve the confusion.
Ancient Greek had a set topic-focus structure, so it is indeed plausible that grēgoreite is a call back to Matth. 24:41-43 (NIV)
“Two women will be grinding with a hand mill; one will be taken and the other left.
“Therefore keep watch [Γρηγορεῖτε active present imperative], because you do not know on what day your Lord will come.
“But understand this: If the owner of the house had known at what time of night the thief was coming, he would have kept watch [ἐγρηγόρησεν active indicative aorist] and would not have let his house be broken into.
Thus Ellicot's commentary on Matth. 26:41: "The first word is eminently characteristic of our Lord's teaching at this period”. Except that Greek is of course not characteristic of the king of jews. Between קָם, (kam) and ע־ו־ר (ʿ-w-r, he.WT) העיר, נעור, הִתְעוֹרֵר “wake, awaken, wake up” etc. I guess it's notable that ἐγείρω ~ part. -ορόων is matching the root structure to a tee. This may be coincidence, but German wartet is a suitable translation
cf. Sternwarte "observatory", En. warden, guard,
cp. Dutch and Low Saxon wachten “to wait”, reflexive, archaic zich wachten "oppassen”: Wacht u voor de hond! (“beware [you] of the dog”, “protect yourself from ...”).
Warten and Wachen have different etymologies, of course. The Greek etymology on the other hand is supplied by Frisk (lsj.gr): "The perf. ἐγρήγορα resembles Skt. jā-gā́ra, Av. ǰa-gāra ‘I am awake’ from *h₁g(r)e-h₁gor- (-᾽γρ- from the aorist ἐγρέσθαι?).” However, cf. ὁράω “to look”, “One may compare Lat. verē-ri ‘observe scrupulously’, ‘venerate’, though with ablauting stemvowel.”
De Wetten did not have this available to him, anyway.
des Gebetes
“deponent middle”, “the Sept. for הִתְפַּלֵּל”, “the thing prayed for is indicated by a following ἵνα (see ἵνα, II:2b.): ἵνα is more common regarded as giving the aim of the twofold command preceding” [Thayer: προσεύχομαι].
Frisk accepts PIE *h₁eugʷʰ- with εύχομαι, Sanskrit óhate, Latin voveō (English avow), “offenbar ein alter Ausdruck der religiösen Sprache”, but the novel ending -εσθαι remains uncertain.
damit ihr nicht in der Prüfung erliegt
Nota Bene: peirasmon, “the Sept. for מַסָּה, an experiment, attempt, trial, proving; (Vulg. tentatio);” [LSJ] may be a separate problem. Since the Latin vulgate may also mean “trial” [L&S] it's not proof that de Wette was working from Greek and Hebrew. It is assumed instead, in this answer, that the scholarship genealogy extends so far, that the Greek may be assumed implicit in his work.