The splunk automatically detects user's browser's default language and uses it. You can use use a temporary solution of changing the web url as http://host:port/locale/..
However there is also another method of editing Splunk/Python-2.7/Lib/site-packages/splunk/appserver/mrsparkle/lib/i18n.py file on line 333 replacing (changing line 449 didn't help me)
locale = "%s-%s" % (locale, locale) if locale else locale
Save and restart splunk
Splunk provides built-in support for these locales:
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