Simply put it means the ISO demo is that of a localized release for the noted language-based territory & market. But it doesn't have to exclusively mean it's stating the level of localization implemented (usually one needs to install the German ISO demo and see for himself if those krauts have implemented their well-known sterile zero-effort dubbing that they do most of the time
). For example it can be a MULTi (multiple languages in a single release), also never being released in an Englisher's market (Rabbids Go Home comes to mind,
myloch for obtaining the rare German magazine the game was promoted on, due to having MULTi6, but never seeing retail availability). It can also have undeveloped and scrapped MULTi characteristics that a modder needs to hack out and implement (Kao the Kangaroo 3 & Beltion: Beyond Ritual). As said regarding the localization level, the people doing these can do whatever, thus not rarely can English UIs (or whatever quirks left out for the lulz, for example the untranslated swearings to the goverment on the ammo boxes, & other Danish easter eggs in Hitman 2 Silent Assassin) be seen on far-Eastern (or elsewhere) games as a designer choice, while the "gist" of the game is in the language for the given market. But yes, the most usual meaning of those ISO demos is that it contains a game localized to one language and it has been sold to its intended market at the time. All in all don't worry about it, if ye want 'em then obtain 'em, if ye like comparing regional differences and similar stuff then go for it
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