code: def tag_lang_pair(cls, src: str, ref: Optional[str]) -> List[str]:
      machine_tags = []
      src_lang = langid.classify(src)[0]
      ref_lang = None if ref is None else langid.classify(ref)[0]
      if ref is not None and src_lang == ref_lang:
          machine_tags.append(f"lang: {ref_lang}")
      else:
          machine_tags.append(f"src_lang: {src_lang}")
          if ref is not None:
              machine_tags.append(f"trg_lang: {ref_lang}")
  
      if (
          ref_lang is not None
          and ref_lang in cls.POTENTIAL_UNSEGMENTED_LANGUAGES
          and ref.find(" ") == -1
      ):
          machine_tags.append("unsegmented_trg")
      return machine_tags
  
imports: from typing import List, Optional
  import langid
  import logging
label_desc: Use image node_id 1 for calling the VizSeqLanguageTagger obj's underlying member method code with example usage: obj.tag_lang_pair(cls, src, ref) and returns: machine_tags