If you visit https://translations.atlassian.com/ you can both download language packs and help improve the general quality of translations. You will need to login with your Atlassian user account to get access.
For Confluence follow the instructions at https://confluence.atlassian.com/doc/installing-a-language-pack-214860198.html
For JIRA follow the instruction at https://confluence.atlassian.com/jira/choosing-a-default-language-185729598.html
I hope this helps.
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