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, block level templates for which there is no equivalent template in the target wiki get copied over to the translation. For example the [[ https://en.wikipedia.org/wiki/Curiosity_(rover) | Curiosity rover article on English]] has an [[ https://en.wikipedia.org/wiki/Template:Infobox_spaceflight | Infobox spaceflight template ]] which is missing in Spanish. When translating the Curiosity rover article from English to Spanish, the original English template is added to the Spanish translation (as shown below). The template name is shown in red text without any of the original information being visible, but it can be inspected showing the parameters are filled. This can be confusing and make the tool look broken.
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# Proposed solution
When a block template does not have a corresponding template for the target language, it should not be added to the translation. This combined with the warnings for template adaptation (T192271) should lead the user to a less confusing situation and reduce the chances of the wrong template to leak into the translation.
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* "Add new template" action will open the new template dialog in order to insert a new template to the translated paragraph, in the same way as if the user accessed the option to add a template from the edit toolbar.
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).