def fast_tokenize(text, ln, tokenizer, mark=True):
  """
  Tokenizes a text given a language and a tokenizer. In addition to the tokenization provided by the passed tokenizer:
  - lowercases the text;
  - replaces all digits with 0s;
  - marks tokens with a langaguage marker ("_ln").
  Returns a list of strings.
  """
  text = text.lower()
  text = ''.join("0" if c.isdigit() else c for c in text)
  tokens = tokenizer.tokenize(text)
  if mark:
    for i, t in enumerate(tokens):
      tokens[i] = t + '_' + ln            
  return tokens