Templates are not available in all languages, and we want to communicate these issues to the user (T192271). In addition, we want to provide a default behaviour that is useful and creates the least confusion.
Currently templates in the source article for which a corresponding templates do not exist in the target wiki, are copied verbatim. For example, the 2nd paragraph of Curiosity (rover) from English into Spanish has the [[ https://en.wikipedia.org/wiki/Template:Curiosity_Mission_Timer | Curiosity Mission Timer template ]] (calculating the number in the "2183 sols" text of the example) which is a template only available in English, but it is copied into the translation as "Plantilla:Curiosity Mission Timer". A similar issue happens for the [[ https://en.wikipedia.org/wiki/Template:Time_interval | Time interval template ]] in the same sentence.
# Proposed solution
When an inline template does not have a corresponding template for the target language, it should be added to the translation as a plain text version of the rendered template to avoid losing the information.
It is important to avoid adding HTML tags and attributes since these could pollute the final wikitext. So let's make sure the content is added as plain text. It is preferred to lose the styling (and even some content) in the process, rather than producing dirty wikitext.
The added content will be highlighted a part of the issue card shown for the user to review for the user to review and adjust:
Note that this proposal is only for those cases where we cannot find a corresponding template. If we can find the right template, it will be added to the translation (even if the mapping of all the parameters could not be complete).