Bug? Date calculation in table fail with locale "de"

["Ulrich J. Herter" <ujh@posteo.de>]

Dear Orgers,

Is this a bug? I'm trying to do some date calculations in a table,
however, when on German language settings I'm getting the following:

| <2016-07-07 Do> | <2016-07-08 Fr> | #ERROR |
#+TBLFM: $3=$2-$1

Debugger output:
Substitution history of formula
Orig:   ?
$xyz->  $2-$1
@r$c->  $2-$1
$1->    (<2016-07-08 Fri>)-(<2016-07-07 Do>)
        --------------------^
Error:  Bad word in date: "Do"

There is no difference when using "date($1)...".
When doing "date" on each individually it works as expected. 

| <2016-07-07 Do> | <2016-07-08 Fr> | 736152 | 736153 | 1 |
#+TBLFM: $3=date($1)::$4=date($2)::$5=$4-$3


Also, putting the locale to LANG="us_US.UTF8" and the dates in US
format, I do NOT get the error and the examples run fine.

My emacs version is GNU Emacs 24.5.1 Org version 8.3.4 (release_8.3.4-
99-ga8e4a3).

Any ideas how to fix this?

Thanks
Uli