Sub items on the "Linked Issues" drop-down are not localized. We had initially thought this was due to the environment being initially created in English.
Further testing was carried out on 2 Jira Cloud trial instances (Spanish and Brazilian Portuguese) that confirm these strings are in English, even if Chrome is set to each language while the trial environment is created.

These strings were not located in Jira Cloud BE and FE projects or on the global search tool.
Note we have received a customer feedback for Jira Server on ticket LF-470 that confirmed this is also the case for Jira Server.

Please see screenshots attached for the ES and PT-BR trial instance of Jira Cloud, and for the PT-BR Server instance of Jira.

All languages affected.