Why it can not find the package from subfolder

I met a strange problem in python package. I have a python package named manchinetranslator in which I created a subfolder for unit tests as shown below.

manchinetranslator/translator.py
manchinetranslator/__init__.py
manchinetranslator/tests/tests.py

In translator.py, two functions are defined as:

def english_to_french(text):
...
def french_to_english(text):
...

The init.py is:

from . import translator

The tests.py contains unit tests of the two functions in translator.py and is as follows:

import unittest
from translator import english_to_french, french_to_english
class Test_english_to_french(unittest.TestCase):
...   
class Test_french_to_english(unittest.TestCase):
...

But when running tests.py, it gives an error as follows:

Traceback (most recent call last):
  File "C:Pythonmanchinetranslatorteststests.py", line 3, in <module>
    from manchinetranslator.translator import english_to_french, french_to_english
ModuleNotFoundError: No module named 'manchinetranslator'

However, when tests.py is put in the same folder as translator.py, it works fine. I guess it may need to set $PYTHONPATH, but I am not sure how to do it. So I need help with this problem.