By default, Jenkins uses the default language of your browser, if it is set. To change it, you could do the follow.
1. Download and install the locale plugin
1.1 From the main page of Jenkins, go to Manage Jenkins -> Manage Plugins
1.2 Click on Availables
1.3 Check the "Locale plugin" and clicks on "Download now and install after restart".
Jenkins will download the plugin and restart if not job has been scheduled
2. Set the language
Once the plugin has been installed, change the language using following steps
2.1 From the main page of Jenkins, go to Manage Jenkins -> Configure System
2.2 Under Locale, there will be a field called "Default Language". Enter the new language. It could be "en" or "ENGLISH".
2.3 Under the text box, check the checkBox called "Ignore browser preference and force this language to all users".