Whenever you create a new chroot, the only locales generated by locale-gen are en_US.UTF-8 and de_DE.UTF-8. However, as some devs (like me) may use other locales in their main systems, then it occurs an error with the chroot locale setting (or is this a feature?):

(main)$ locale
LANG=es_CL.UTF-8
LC_CTYPE="es_CL.UTF-8" 
LC_NUMERIC="es_CL.UTF-8" 
LC_TIME="es_CL.UTF-8" 
LC_COLLATE="es_CL.UTF-8" 
LC_MONETARY="es_CL.UTF-8" 
LC_MESSAGES="es_CL.UTF-8" 
LC_PAPER="es_CL.UTF-8" 
LC_NAME="es_CL.UTF-8" 
LC_ADDRESS="es_CL.UTF-8" 
LC_TELEPHONE="es_CL.UTF-8" 
LC_MEASUREMENT="es_CL.UTF-8" 
LC_IDENTIFICATION="es_CL.UTF-8" 
LC_ALL=

(chroot)$ locale
LANG=es_CL.UTF-8
LC_CTYPE="es_CL.UTF-8" 
LC_NUMERIC="es_CL.UTF-8" 
LC_TIME="es_CL.UTF-8" 
LC_COLLATE="es_CL.UTF-8" 
LC_MONETARY="es_CL.UTF-8" 
LC_MESSAGES="es_CL.UTF-8" 
LC_PAPER="es_CL.UTF-8" 
LC_NAME="es_CL.UTF-8" 
LC_ADDRESS="es_CL.UTF-8" 
LC_TELEPHONE="es_CL.UTF-8" 
LC_MEASUREMENT="es_CL.UTF-8" 
LC_IDENTIFICATION="es_CL.UTF-8" 
LC_ALL=

This is what happens in my case. The problem with this is that as the chroot doesn't have the correct line in /etc/locale-gen to include my system locale in the chroot, then sphinx-build falls into an error (this is what happens when trying to build the linux-libre documentation):

Traceback (most recent call last):
  File "/usr/bin/sphinx-build", line 11, in <module>
    load_entry_point('Sphinx==1.8.1', 'console_scripts', 'sphinx-build')()
  File "/usr/lib/python3.7/site-packages/sphinx/cmd/build.py", line 313, in main
    locale.setlocale(locale.LC_ALL, '')
  File "/usr/lib/python3.7/locale.py", line 604, in setlocale
    return _setlocale(category, locale)
locale.Error: unsupported locale setting

I solved this by adding es_CL.UTF-8 UTF-8 to /etc/locale.gen to the chroot, and then running locale-gen. However, if there is a way to make the locale use "C" or one of the default locales, it would be better IMO (I think in the past is used to use "C", right?).