We have several tickets where customers report that the language to be translated is shown instead of the original language. Or the original language was shown instead of the translated one.
It turns out that this is caused by wrong proxy settings.
If you use IIS: There is a ^-character used in the match url field,but it shouldn't (see specific notes chapter in https://confluence.atlassian.com/kb/proxying-atlassian-server-applications-with-microsoft-internet-information-services-iis-833931378.html)
If you use Apache: Use an enclosing "/" for ProxyPass and ReverseProxy (see https://confluence.atlassian.com/kb/proxying-atlassian-server-applications-with-apache-http-server-mod_proxy_http-806032611.html)