User wants to see the interface in the language that is specified in the settings. It is obvious.
I just checked this issue in other languages.
Cause of the problem is the collision between user language preferences and "target" field of the query string.
Example:
user logging on to the my.avast.com this way:
___ id.avast.com/ja-jp/login?target=https%3A%2F%2Fmy.avast.com%2Fja-jp%2F ___
Now user appears on the japanese version of the site (my.avast.com/ja-jp).
Even if the settings specified another language.
How can user get this link? It remains from the previous user who just log off.
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Replace japanese with your local language and try to use english version of the site as I do 
