[Feature] Add a way to filter archive content by language ?

[cderv]

It could be interesting to be able to add a tag for langage to help discover new book.
It could be added manually by authors in the YAML header or estimated base on title and description.

What do you think ?

[yihui]

I thought about it in #8, and implemented it via matching the description against a custom tag list: https://github.com/rstudio/bookdown.org/blob/master/R/tags.txt which generates this page: https://bookdown.org/home/tags/ (not very useful IMO but better than nothing).

As I said in #8, it will be nice to let authors provide their own tags, but it seems to be more work for me to do in bookdown, and I didn't have time for it.