text: Title: Pipeline is incorrect with specific lang in MultilingualPipeline if lang_config is set
  Body: **Describe the bug**
  If I set lang_config in MultilingualPipeline for specific language , the Pipeline is always initialized as "en"
  
  **To Reproduce**
  `lang_configs = {'hi': {'processors': 'tokenize,pos'}, 'ar': {'processors': 'tokenize,pos'}}`
  `nlp = MultilingualPipeline(lang_configs=lang_configs)`
  
  If I put hindi/arabic texts as input, the language identification works well, but the Pipeline is initialized with "en".
  The problem seems that the "lang" param is not set in https://github.com/stanfordnlp/stanza/blob/main/stanza/pipeline/multilingual.py#L68
  
  If I set lang explicitly in lang_configs, it works as expected, but I think this should be fixed to avoid misleading.
  
  **Expected behavior**
  Pipeline should be init with specific lang even if 'lang' is not set explicitly.
  
  **Environment (please complete the following information):**
   - OS: [e.g. Windows, Ubuntu, CentOS, MacOS]
   - Python version: [e.g. Python 3.6.8 from Anaconda]
   - Stanza version: [e.g., 1.0.0]
  
  **Additional context**
  Add any other context about the problem here.
  