Translations are handled by language packs, which are often incomplete, especially when Atlassian has no native speakers with the time to put into it.
They encourage us to translate and contribute back into the language packs. I'm sure they would welcome your translations for the phrases that are missing.
https://confluence.atlassian.com/adminjiraserver074/translating-jira-881683323.html is the starting point. It allows you to create your own language packs too, if you don't want to wait until your translations make it into an official one.
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