Passing locale parameter to Checkout page

[turczyn_lukas]

Hi, we have a problem in out theme. I'm pretty sure that this could be problem with some settings. 
Problem:
We have 4 languages published in shop and also we use Weglot with same languages. Translatations works great except checkout page where we always see Default German language.

What we know:
- When we are adding &locale=en to url in checkout it's translated but somehow this locale property is not added when we click Check-out button in cart.
- Url in cart form also looks fine because url in <form> attr is /en/cart or cart?locale=en (depends on settings) but none of these is working. Locale parameter is still missing.
- We also tried to remove all languages in Shopify and left settings only in Weglot as one of discussion in community mention but this also not fixing issue
- One thing concern me that in Checkout settings I see only one language that checkout is available (screen) and also when I click "Menage checkout language" I'm going to edit language settings in theme that doesn't have any select to choose different language to change.

I hope that this could be some issue with settings of markets or languages.
Thanks in advance.