ProMods

In the cities of Spain, attached to the walls of supermarkets as Día or LIDL, advertising posters appear in English ("50% off" ). They should be in Spanish ("50% de descuento") or, if you want real realism, in Catalan in Barcelona or in Basque in Bilbao or Donostia. But not in English.

It's have sense

This is not just found in Spain, it's everywhere. The problem is that these texts are fixed on the model, thus requiring a new model to be made, which is time consuming and I guess not worth it, considering it's just a small detail.

I'd say it is worth fixing as it might break realism.

Well I know from internal side that it's very time consuming to add new textures to the map editor (e.g. the tourism signs on the autobahn) and on models even more
Soo I would say just let it be like it is now and replace/rework them later on when they are out of work or undertaxed

The process is quite involved to just change some little bit of text. Since these models are pretty much images that have been customized as textures on a sign model, your going to have to redo the original image to start with on something like Photoshop and/or Inkscape which is already time a process that is quite involved. you have to either delete that text entirely, which might include some of the background image if it was placed upon it, or you have to come up with a new image entirely with that text applied to it. I'm not sure if the devs really want to invest so much extra time just to change the the text from English to a native language, which they might have to do for each country that uses that text. There is really a point where trying to replicate a totally realistic environment becomes less advantageous as you are spending all that extra time away from expanding the map just for little details here and there.

It'll be probably very difficult for ProMods to add translations for Advertisements for every country, not to mention if the text is part of the textures, they would have to rework the def .sii files, and rework the pmd, pmg, pmc and the .tobj, .mat, and any .dds files for the textures of every translation. Also, the advertisements themselves might have to be adjusted by country as prices and discounts may vary by country, for example, although a McDonalds cheeseburger may be half-price after 8pm on Mondays in the U.S., whereas in Canada, that same cheeseburger at McDonald's may not be half-price there and vice-versa.

I think it's the half full / half empty thing. I'm already very glad that there is Día in Spain and not just some generic supermarket. Anyone who has done some modding (which I did for a Train Simulator) knows how time-consuming changing parts of templates is - so I'd rather op for having Madrid in 2.40 with 50% off signs than Madrid in 2.60 with 50% de descuento.