DOC: set the documentation language
Ensures the language is specified in the generated HTML. [Documentation on the configuration option.](https://www.sphinx-doc.org/en/master/usage/configuration.html#confval-language)

Before:

```
$ curl -s https://pandas.pydata.org/docs/dev/getting_started/comparison/comparison_with_spreadsheets.html | grep "<html"
<html>
```

After:

```
$ python make.py --single getting_started/comparison/comparison_with_spreadsheets.rst
$ grep "<html" build/html/getting_started/comparison/comparison_with_spreadsheets.html 
<html lang="en">
```

- [ ] ~~closes #xxxx~~
- [x] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [ ] ~~whatsnew entry~~
