I did the same for German adding "lang": "de-de" to the config. After that I had the same situation as you - Mycroft got stuck during booting. After looking into logfiles I found that several localization resources for skills etc. were missing (in my case for “de-de”).
As a quick&dirty solution I copied all missing files from the “en-us” folders into the “de-de” folders. This will get you over the stuck “Download settings” but some skills/intent will not react to prompt in your language as the intent is available in English only.
You can look into Skills-Logfiles by sudo journalctl -f -u dinkum-skills
There was also a file from /opt/mycroft-dinkum/shared/mycroft/res/text/de-de missing: noise_words.list - I simply created a empty file with that name.
You may also want to change the configuration for STT and TTS to your language. For STT i have installed VOSK (using vosk_stt_plug plugin), for TTS I use Mimic3 with a german voice.