write_html: locale config does not work

Hi!

I'm using plotly to generate a standalone HTML-file that contains only my plot. I do want the axes to be localized in German (one axis has Dates).

This code does not work:

fig.write_html("index.html",
  include_plotlyjs="cdn",
  config={"locale": "de"}
)

The reason is that the following line is missing in the generated HTML-file (cf. plotly.js documentation)

<script src="https://cdn.plot.ly/plotly-locale-de-latest.js"></script>

I suggest to add a locale argument to the write_html function that does two things (think of de as exchangeable):

• it adds the missing localization file somewhere here
• it adds "locale": "de" to the plotconfig

Start in packages/python/plotly/plotly/io/_html.py around line 345 and trace how write_html builds the standalone HTML and plot configuration. Verify the example with a German locale, then ensure the generated document loads the locale script and applies the requested locale to the plot configuration.