In some cases this is just an unspoken assumption because the blog posts are looking at, say, Firefox, and Firefox supports a lot of languages. In other cases, it's framed as a moral or professional imperative -- you're a bad developer if you don't consider the needs of Arabic users, if not a bad person. (I'm not sure how I feel about the globalist/colonialist worldview that seems to drive this idea that Arabic users won't have any software if Silicon Valley doesn't write it for them. I guess if you work at a big global megacorporation, it's your job to write big global megasoftware.)
But yeah, if you aren't trying to do that, then text is a much easier problem than these posts describe. Tons and tons of embedded devices, video games, and so on get away with having much simpler text rendering and layout code that only supports a few languages. In English, you really can start with a very simple layout system, then add features like kerning and ligatures and hyphenation and watch the text get progressively nicer-looking!