Use first_native_name in LanguageDropDown.vue instead of native_name

[jayoshih]

https://github.com/learningequality/studio/blob/hotfixes/contentcuration/contentcuration/frontend/shared/views/LanguageDropdown.vue#L105

Should be using first_native_name as native_name can be a comma separated list of names.

[cerdo03]

Please review my pr.

[MisRob]

Thanks @cerdo03!