You can now get the currently selected language inside a field type plugin with sbLanguage. Example:

const Fieldtype = {
  mixins: [window.Storyblok.plugin],
  template: `<div>Language: {{ sbLanguage }}</div>`,
  methods: {
    initWith() {
      return {
        plugin: 'example_plugin'
      }
    },
    pluginCreated() {
      console.log(this.sbLanguage)
    }
  }
}