When I disable translation language fallback, then key translations are displayed instead of a particular translation. However, when running with SSR mode, translation keys are not displayed at all.
I am using the latest version of Spartacus and running server by npm run build:ssr && npm run serve:ssr.
Is there any way to have missing translations displayed in prod mode with enabled SSR?
It's a correct behavior. If you need a custom fallback behavior, please overwrite this method: https://github.com/SAP/spartacus/blob/4432a2bed0c6cf04aa2ff412792f0e8cb24686a3/projects/core/src/i18n/i18next/i18next-translation.service.ts#L70
SSR runs the production version of the application. In development mode missing keys are rendered in the HTML (and a warning appears in the console). But in production mode missing keys are rendered in the HTML as a non-breaking space is displayed. See https://sap.github.io/spartacus-docs/i18n/#fallback-language