This MR is aimed to simplify SlapOS test zuite involving multi-language(Chinese and English in this case).

Thanks to Rafeal who came up with Base_translateString() function, making it possible to use localizer translation messages directly. And Thanks to Xiaowu's suggestion to use getDefaultAvailableLanguage(), so we can get/set a global variable of language in one macro.

Since our SlapOS test zuite is involving multi-language, set_default_available_language became obligatory to use especially when the test is applied with a macro using Base_translateString() function.

Additionally, removed slapos_jio_zh_test bt5 for good.

Last but not least, define "TEST-TITLE" as ${title} in Selenium so we can apply one message to customized TEST-TITLE in Localizer.