Select: Add default value to loadingText

[obregonia1]

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According to Document, default value of loadingText is 'Loading'. But that default value is not exist.

[wangdaodao]

其实他是设置了默认值，可以看下：

https://github.com/ElemeFE/element/blob/2c4ac8099c7c04e1b89a6d31f3d3fbc01334a971/packages/select/src/select.vue#L208

https://github.com/ElemeFE/element/blob/2c4ac8099c7c04e1b89a6d31f3d3fbc01334a971/src/locale/lang/en.js#L62

https://github.com/ElemeFE/element/blob/2c4ac8099c7c04e1b89a6d31f3d3fbc01334a971/src/locale/lang/zh-CN.js#L62

这里他用了国际化处理了默认文本，感觉这样处理比加默认的会好，假如现在加了默认值，就不会走后面国际化了，所以还是不建议这么修改。

[obregonia1]

@wangdaodao Thanks!! Is that means setting default language to English better?

[wangdaodao]

I think it would be better to set the default language.