Rav Hirsch translates (actually his grandson translated into English from the original German)
And Melchizedek King of Salem had brought out bread and wine but
he was also a priest of the most high God.
Rav Hirsch explains that this is to set him apart from the King of S'dom who came out with demands on Avram. The translation that you give above shows that unlike the King of S'dom Melchizedek acted properly and with graciousness as befitted a priest of Hashem. Rashi points out that he had done this even though his own sons had been killed in the fighting.
bread and wine: This is done for those weary from battle, and he [Malchizedek]
demonstrated that he bore no grudge against him [Abram]
for slaying his sons (Tan. Lech Lecha 15). And according to the
Midrash Aggadah (Gen. Rabbah 43:6), he hinted to him about the meal
offerings and the libations, which his [Abraham’s] children would
offer up there.
It appears that the translation that you cite puts the but in the beginning of verse 18 in order to emphasize the difference between the two. Indeed, the behaviour of the King of S'dom shows what led to the destruction of the city and its surroundings.
Rav Hirsch explains:
These two verses show a striking contrast. The King of Sodom, who must
have felt very humiliated, and still more deeply indebted towards
Abraham, not only had allowed him to undertake the pursuit alone with
his little band without joining him, but after the victory had been
won, comes out to meet him entirely as a King, thinks it is already a
sufficient honor for Abraham to be met on equal terms as King to King
"in the valley שוה". And still he comes to beg. To ask for favours
[sic], to "demand", is what a King of Sodom uderstands, but to refresh
the wornoutfamished victors with a piece of bread, with a drink of
wine, does not enter the head, forms no part of the code of manners of
His Majesty of Sodom! On the other hand Melchizedek, King of Salem,
who really had nothing to do at all with the affair, had bread and
wine brought out, but then he was also a priest to the Highest God. So
that even among the polytheistic nations to conception of an
All-highest God, אלוקא דאלהא, a God of gods had not become entirely
lost.
