In Odoo18 (and maybe from Odoo16 on) you don't have to use "Import translations" to load translated data. It seems reserved for translation of software interface.

Use instead the model where you want to load the data (for example Product) and switch your environment to the language you want to upload (for example nl_BE). This language had to be activated beforehand in Settings.Translations.Languages (provided you are in Debug mode)

Don't forget to use the right external ID in order to update instead of create new objects. This external ID can be obtained by a previous export (when selecting " I want to update data (import-compatible export)". 

Those exports/imports support Xslx file format also.

Other fields than the translations can be uploaded at the same time. You have to handle every language separately in sequence.

Many thanks to Ray Carnes to give me the solution.

I deleted the conversation by mistake. So I post a summary for further use.