Can't answer most of your questions, but in "Goodbye Dolly Gray", Rayne Kruger's account of the Boer War, Kruger makes the claim that neither world war had the impact on the British public that the Boer War did. After a long period of peace, coupled with the belief in England's superiority over the rest of the world, the new technology of the telegraph provided a relatively immediate report of the initial stages of the war, which came as an immense shock to the country. Long lists of the dead were for the first time reported in the newspapers, and the 'gutter press' went berserk. Stories could be personalised, and there's an account of a 14-year old child soldier, John Dunne, who was wounded and lost his bugle in the Battle of Colenso. When he returned home, he was met with a huge crowd, and presented with a new bugle. There was some criticism (questioning what child soldiers were doing on the battlefield, the concentration camps, etc), but on the whole, the British public was overwhelmingly in favour of the war, and many rushed to enlist. Most criticism was aimed at the incompetent planning and tactics in the war, not on any humanitarian grounds. The Khaki election was fought almost entirely on the single issue of the war, and the Conservatives won overwhelmingly.

Outside of Britain, much of Europe was strongly on the side of the Boers, and it was apparently a revelation to Britain how strongly the country was loathed

First off, Pakenham is a great historian. A little dense at times, but his books are always well researched. Kudos to you for giving him a read.

Now, a few relatively short answers to your questions, and some book recommendations for you:

What effects did the war have on language(s) & culture(s) of the British Empire, South African Republic & Orange Free State?

The Boer-British relationship is quite complex. It was an annexed imperial territory (originally Dutch), thus there was - at the start - a cultural rift between true 'Boers,' British administrators, and British settlers. This rift reinforced the Boer identity, especially since the Boers viewed themselves as the buffer population between heavily-British Johannesburg and the 'frontier,' which was populated by the Zulu and other African states/cultures. The wars, therefore, solidified the view of many Boers that their feeling of difference was justified, and that British rule was unilateral. If anything, therefore, Boer culture came out of the wars more sharply defined as being unique to, and - critically - exclusive to the non-British, white population of southern Africa.

On the flip side, the Boer Wars were closely followed and widely publicized at home, thus (I'd argue) that the war was more impactful on British culture than on Boer culture, at least in the sense that it was more divisive. The ideals of 'Greater Britain' (i.e. the concept of a unity of Britishness under the banner of British imperialism) collapsed under the weight of the Boer War, particularly when news of the detention camps and brutal morale tactics employed by the British Army were revealed to the public. Furthermore, the limits of British power on land were tested during these wars, a fact that, while beneficial to the British Expeditionary Force during WW1, was catastrophic to military morale at this time.

Books: Anything by John Mackenzie, especially Propaganda and Empire. Also the Idea of Greater Britain by Duncan Bell.

When WW1 was on the horizon was there any attempt to avoid another war soon after such a costly one?

Oh sure, and for many of the reasons you probably picked up from Packenham: modern war was messy, and the amount of violence produced by modern rifles and machine guns far outclassed anything that had been experienced by British soldiers up to this point. It is worth noting that, from 1815 up to the Boer Wars, most British military engagements had been fairly 'irregular' and were not, as a result, fair tests of the potency of industrial warfare.

Book: Ian Herndon, Britain’s Forgotten Wars: Colonial Campaigns of the 19th Century

Was the Boer War one of the reasons, Britain decided to follow a policy of "splendid isolation"? Because they were preoccupied with other things?

Splendid isolation was more of a post-1815 policy (and has, mind you, largely been abandoned as a historiographic concept), and had passed out of favor by the time the Boer War came about.