Yeah, had issues with the German version, too.
Even after uninstalling that and installing the en_UK version, parts of the program were still in German and I got funny error messages galore...
What helped me was to uninstall V15 yet again and then to make sure I deleted EVERYTHING concerning V15 afterwards, including the registry keys in HKEY_CURRENT_USER\Software\Xara\Xtreme Pro x64\15.
If you once installed V15 with another locale, there will be an entry left in HKEY_CURRENT_USER\Software\Xara\Xtreme Pro x64\15\Options\Install\Language, which doesn't get updated even when you reinstall the software with language set to English (GB). This seems to cause trouble with V15 not knowing what language it's supposed to use, resulting in some weird mixed language version.
After I cleaned the registry of any trace from V15, another reinstall went well and even the content downloader was automatically set to the right language. Most of my issues are gone now.
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