Actually, you can change it without modifying the source code. It is a bit hard though for "Issues" bacause it is used in so many places. But we have done that for customers. Two options:
1. Using the "In Product Translations" plugin (Too many places to change though) : https://marketplace.atlassian.com/plugins/com.atlassian.translations.jira.inproduct
2. Translating JIRA.
This documentation may help you: https://developer.atlassian.com/display/JIRADEV/Customizing%20Text
Using the translator plugin that Jobin has mentioned is a good option to achieve this.
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