It looks like there is an issue with the "en" files.
Anders mentioned this in his blog post as well:
How to change the texts/translations on a non-English Self-service Portal
Quote from Anders:
"Note: This doesn’t seem to work for the original texts written in English even though there is an English directory (C:\inetpub\wwwroot\System Center Service Manager Portal\ContentHost\Clientbin\en)."
Andreas Baumgarten | H&D International Group