I'm using Mylyn 3.17 for current Eclipse and Spring Tool Suite 3.7.2 with the JIRA connector "Tasktop Atlassian JIRA Connector 4.4.0.v20150930-2223-RELEASE".
If the project of the issues I receive from JIRA as result of a query has english as language configured, everything is fine and the icons in the task list view are different for different JIRA issue types (bugs, improvement, task etc.). But if the project has any non-english language configured, all issues are shown with a neutral icon in the task list. This is the same, the UI Legend window shows for 'Task' . The reason for this behavior seems to be, that Mylyn try to parse the attribute issue type sent by JIRA for every issue as string/text. For a non-english project, JIRA is sending the translated descriptions for the issue type ('Fehler', 'Verbesserung', 'Aufgabe' etc. for e.g. German) and those values seems to be unknown for Mylyn and therefor the neutral 'Task' icon is shown for all items in the list. You can see the values JIRA is sending as issue type name within the hover for every item on the Mylyn task list.
Kind regards Dominik Hirt