At the present time, it is not currently possible to change the language setting for this specific integration. While you can choose your own Atlassian Cloud language preference, as explained in Manage your language preferences, doing so will not help in this app.
So I created a feature request for this over in, JRACLOUD-76231 : Microsoft Teams integration for Jira Cloud should support user selected language. I would recommend watching/voting on this issue to help our team prioritize new features.
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