To change user interface language in libreoffice:
Make sure the language pack is installed.
For this,open "Language & region " in system settings.
Click on comple install under "install missing languages" ,if the required language is not available.
Note: No need to change any other settings in that tab.
libreoffice tools-->> options.. -->> language settings -->> language.
3.Now change User interface, Locale setting , Default currency , and default document language to desired language.
This will change the language only for LibreOffice, leaving other applications in English.