[Edit]->[Add]->[Timer]->[(Upmost element)] shows the word of user's default locale. Even if you change the language property in jmeter.properties, The word doesn't change accordingly (For Spanish, it does). I'm running JMeter in Japanese locale. When I set language=en at jmeter.properties, I still see the element is written in Japanese (’蔃Xƒ‹[ƒvƒbƒgƒ^ƒCƒ}j. The same goes for the most of locales provided by [Options]->[Choose Language] except for Spanish and traditional Chinese. (While setting zh, zh_CN or zh_TW to jmeter.properties makes no effect. Only way to change to Chinese is by [Choose Language] menu above.)
I've checked in a change to allow the language to be specified with a country variant - e.g. language=zh_TW. Not all properties have been translated; ones that have not been translated are shown in the default locale.
This issue has been migrated to GitHub: https://github.com/apache/jmeter/issues/1663