I went to Nagasaki! While I was there I bought a modern reprint of Sakamoto Ryōma's Kaientai's famous Waei tsūin iroha benran 和英通韻伊呂波便覧 ("Basic manual of Japanese-English transliteration").

Here's how the kanbun introduction starts, with a translation by me:

自官許外交開港通商以来。東西諸蛮。雲集雨至。千艦万船。去来無間断也。是所以徳化之及遠也。誰不仰之哉。当是時。国人接蛮客。不可不以言語與文字。言語不通。文字不弁。則貨物運輸之際。恐生謬誤。謬誤生。則国家害亦曰不生乎。

Since the ministers allowed foreign relations, opened the ports and allowed merchants in, barbarians [諸蛮] from the east and west have gathered like clouds and arrived like rain, a thousand ships and myriad boats arriving without end. This is because virtuous deeds travel far. Who will not marvel at this? At this time, when our countrymen have contact with barbarian visitors [蛮客], they have no choice but to use speech and writing. If they cannot communicate in speech, if they are not adept in writing, this will surely cause errors to arise when trading cargo. If such errors arise, can one say that harm to the nation will not thereby arise too?

It also comes with a modern Japanese translation, of which I will share (and translate) two sentences:

お上が諸外国との交流を許し、港を開いて貿易を認めて以来、東西の諸国が我が国を目指すようになった。 [...] こんな時、我が国の人が外国人と接するとき、言語と文字でもってするほかはない。

Since our leadership allowed interaction with foreign countries, opened the ports and permitted trade, nations [諸国] from the east and west have set their sights on our country. [...] At a time like this, when the people of our country have contact with foreigners [外国人], they have no choice but to use speech and writing.

Ryōma The anonymous author sounds a lot more tolerant!

The issue of course is whether 蛮 for Ryōma the author had the same connotations as "barbarian" (its usual contemporary translation) does for us. That is, you could argue — and I think credibly — that the use of 蛮 was meant as an objective descriptor rather than an unjustified insult, even if the ultimate meaning was "barbarian".

On the other hand, it's also true that there were other words for "foreign country". The Sino-Japanese contemporary standard, 外国, is attested as far back as the 927 CE Engi shiki in the Nihon kokugo daijiten's earliest citation. For "foreigner", 外国人 had been available for nearly as long.

So while Ryōma the author might not have been going out of his way to insult the foreigners newly allowed into Japan, nor did he go out of his way to refer to them respectfully. He used 蛮 twice — and he uses it several times more before the two-page introduction is complete — he knew what he was doing. I do not think the case for translating his choices into more politically correct terminology for the present day is very strong.

(If you read Japanese, you can learn more about the book, including some small photographs og pages and a full yomikudashied version of the intro, at this post on 東書文庫通信. Waseda University have also put a very similar book online; pages 4-29 appear to be completely identical.)