Timestamp insertion produces timestamp with localized day

[schoettl]

The shortcuts \sa and \si produce a timestamp with a localized day, e.g. German <2016-07-29 Fr>.

But it should always be english, because:

1. only engish timestamps are syntax highlighted
2. ~~the speeddate plugin to increment dates only works with the english format~~

My environment variables are:

$ env | grep -E 'LANG|LC|LOCALE'
LANG=en_US.UTF-8
GDM_LANG=de_DE.utf8
CSF_LANGUAGE=us

[schoettl]

I just found that emacs' orgmode also uses the user's locale but they have a setting to change this: http://orgmode.org/worg/org-faq.html#timestamps-and-system-time-locale

I guess it's hard to implement such a setting in vim-orgmode. I just tried to output the engish format but output is only one side of the coin. Parsing is the other.