I think custom scenarios can only have ONE original text, hence it wouldn’t be possible to translate them. It’s not like in the campaign senarios, which work with a separate strings file for each language.
It’s a game limitation, I believe. Normal to happen, also in other games such as classic WarCraft 3 (not Reforged), which only supported one original text for their custom map descriptions.
If players want to use these maps for multiplayer games, they have to share the same file, so I think it makes sense that there is no translation for these scenarios.
You probably don’t know this translation system. In General, translation into different languages is done as follows:
String files are created in the languages that will be used, for example, English, French, German, and Russian (as an example)
In the program of the game recorded number (ID) of the row that refers to a line in a file the selected language (say, 34855, with the English string it will be the word “Gold” (for example), and Russian strings - “Золото” (Russian Gold). And if you select English, it shows Gold, and if Russian - Золото.
In this case, ID = -1, that is, the translation does not refer to any string in the strings file, but the description is written in one language (what is written in the window itself under the phrase “ID строки”. So you can just add an ID for each description and everything will be fine)