You will get an error:
Translation search server unavailable:Result window is too large, from + size must be less than or equal to:  but was . See the scroll api for a more efficient way to request large data sets. This limit can be set by changing the [index.max_result_window] index level parameter.
In CrossLanguageTranslationSearchQuery one could add a check if offset > 10000 then stop (or even better: catch the exception so that we do not hardcode the limit), show what has been found, and then show an error message. Some code needs to be changed to indicate the result is incomplete.
It could also be migrated to use https://www.elastic.co/guide/en/elasticsearch/reference/current/search-request-scroll.html but that could be a bit more complicated.
If you work on this task you must have MediaWiki running with a working Translate extension with ElasticSearch setup. For testing the index.max_result_window should be set to a low value.