Autodetection fails

Autodetection seems to fail, even on your example:

$ translate-cli -t zh "This is a pen."

Translation: '' IS AN INVALID SOURCE LANGUAGE . EXAMPLE: LANGPAIR=EN|IT USING 2 LETTER ISO OR RFC3066 LIKE ZH-CN. ALMOST ALL LANGUAGES SUPPORTED BUT SOME MAY HAVE NO CONTENT

-------------------------
Translated by: MyMemory

Also explicitly asking for autodetection fails:

$ translate-cli -f autodetect -t zh "This is a pen."
$ translate-cli -f 'autodetect' -t zh "This is a pen."

It comes up with the same result, for any target language. Only entering a valid language identifier for source language explicitly instead works as expected:

$ translate-cli -f en -t zh "This is a pen."

Translation: 这是一支笔
-------------------------
Translated by: MyMemory

Trying to use a different service provider fails completely:

$ translate-cli -p deepl -f autodetect -t zh "This is a pen."
Traceback (most recent call last):
  File "/home/demo/.local/bin/translate-cli", line 6, in <module>
    from translate.__main__ import cli
  File "/home/demo/.local/lib/python2.7/site-packages/translate/__main__.py", line 6, in <module>
    main()
  File "/home/demo/.local/lib/python2.7/site-packages/click/core.py", line 829, in __call__
    return self.main(*args, **kwargs)
  File "/home/demo/.local/lib/python2.7/site-packages/click/core.py", line 782, in main
    rv = self.invoke(ctx)
  File "/home/demo/.local/lib/python2.7/site-packages/click/core.py", line 1066, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/home/demo/.local/lib/python2.7/site-packages/click/core.py", line 610, in invoke
    return callback(*args, **kwargs)
  File "/home/demo/.local/lib/python2.7/site-packages/translate/main.py", line 159, in main
    translator = Translator(**kwargs)
  File "/home/demo/.local/lib/python2.7/site-packages/translate/translate.py", line 25, in __init__
    'Please check providers list bellow: {!r}'.format(self.available_providers)
translate.exceptions.InvalidProviderError: Provider class invalid. Please check providers list bellow: ['microsoft', 'mymemory']

Any idea what could cause all this? Maybe the install errors I encountered could cause it?

$ pip install translate
[...everything fine until:]

Building wheels for collected packages: filelock, scandir
  Running setup.py bdist_wheel for filelock ... error
  Complete output from command /usr/bin/python -u -c "import setuptools, tokenize;__file__='/tmp/pip-install-snvHIF/filelock/setup.py';f=getattr(tokenize, 'open', open)(__file__);code=f.read().replace('\r\n', '\n');f.close();exec(compile(code, __file__, 'exec'))" bdist_wheel -d /tmp/pip-wheel-HPe58C --python-tag cp27:
  usage: -c [global_opts] cmd1 [cmd1_opts] [cmd2 [cmd2_opts] ...]
     or: -c --help [cmd1 cmd2 ...]
     or: -c --help-commands
     or: -c cmd --help
  
  error: invalid command 'bdist_wheel'
  
  ----------------------------------------
  Failed building wheel for filelock
  Running setup.py clean for filelock
  Running setup.py bdist_wheel for scandir ... error
  Complete output from command /usr/bin/python -u -c "import setuptools, tokenize;__file__='/tmp/pip-install-snvHIF/scandir/setup.py';f=getattr(tokenize, 'open', open)(__file__);code=f.read().replace('\r\n', '\n');f.close();exec(compile(code, __file__, 'exec'))" bdist_wheel -d /tmp/pip-wheel-8EKBmy --python-tag cp27:
  /usr/lib/python2.7/distutils/extension.py:133: UserWarning: Unknown Extension options: 'optional'
    warnings.warn(msg)
  usage: -c [global_opts] cmd1 [cmd1_opts] [cmd2 [cmd2_opts] ...]
     or: -c --help [cmd1 cmd2 ...]
     or: -c --help-commands
     or: -c cmd --help
  
  error: invalid command 'bdist_wheel'
  
  ----------------------------------------
  Failed building wheel for scandir
  Running setup.py clean for scandir
Failed to build filelock scandir
Installing collected packages: click, contextlib2, zipp, six, scandir, pathlib2, configparser, importlib-metadata, pyparsing, packaging, filelock, pluggy, appdirs, distlib, typing, singledispatch, importlib-resources, virtualenv, toml, py, tox, cfgv, pyyaml, aspy.yaml, nodeenv, futures, identify, pre-commit, certifi, urllib3, idna, requests, translate
  Running setup.py install for scandir ... done
  Running setup.py install for filelock ... done
Successfully installed appdirs-1.4.4 aspy.yaml-1.3.0 certifi-2020.12.5 cfgv-2.0.1 click-7.1.2 configparser-4.0.2 contextlib2-0.6.0.post1 distlib-0.3.1 filelock-3.0.12 futures-3.3.0 identify-1.6.2 idna-2.10 importlib-metadata-2.1.1 importlib-resources-3.3.1 nodeenv-1.6.0 packaging-20.9 pathlib2-2.3.5 pluggy-0.13.1 pre-commit-1.21.0 py-1.10.0 pyparsing-2.4.7 pyyaml-5.4.1 requests-2.25.1 scandir-1.10.0 singledispatch-3.6.1 six-1.15.0 toml-0.10.2 tox-3.23.0 translate-3.5.0 typing-3.7.4.3 urllib3-1.26.4 virtualenv-20.4.4 zipp-1.2.0

Python 2.7.16
pip 18.1 from /usr/lib/python2.7/dist-packages/pip (python 2.7)

P.S.: Many thanks for this great commandline tool!

Reproduce the autodetection and provider failures with the commands in the report, then start at translate/main.py and translate/main.py, following translator construction into translate/translate.py. Check how the source language and provider are validated and how the available providers are selected. Done means autodetection works for the shown translation and invalid provider handling is consistent with the supported provider list.