Thank you for the screenshot I now see that you are using the cloud version of JIRA which does not allow the upload of JAR files from local copies and as you have seen the version available from translations.Atlassian.com does not match the add-on descriptor. You have two options available to you.
If you click on the link for JIRA Core (assuming that this is the application you wish to have in Czech) you will then see this page.
You should confirm that this is the right product and then select the version that matches your installed version.
This will then give you a jar file as a download. Follow the instructions provided below and you should then have the Czech language installed as a plugin.
The manage addons page on JIRA is found at <yourhost>/plugins/servlet/upm and looks like this.
I tried, but here is message. I have JIRA servicedesk project on your website: We couldn't install the add-on from https://translations.atlassian.com/download/JIRA+Service+Desk/3.1.0-OD-10-001/cs_CZ/JIRA+Service+Desk-3.1.0-OD-10-001-language-pack-cs_CZ.jar. Use an add-on descriptor URL from the Atlassian Marketplace
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