Replace polyglot with langdetect

https://github.com/aboSamoor/polyglot depends on pycld2, which depends on PyICU, which needs C compiler and tons of stuff to build, because it [doesn't ship wheels](https://github.com/ovalhub/pyicu/pull/39#issuecomment-458000833).

The only `polyglot` feature what is used it language detection - https://polyglot.readthedocs.io/en/latest/Detection.html

This can be done with less sophisticated dependencies, such as https://github.com/Mimino666/langdetect