I just ran gimp, and the application runs in English.

No locale is installed.

I just inspected gimp.spec, and everything seems to be correct.

oct:~ # rpm -q --recommends gimp
gimp-help-browser
gimp-lang
gimp-plugins-python = 2.8.16
iso-codes
oct:~ # rpm -q gimp-lang
package gimp-lang is not installed
oct:~ # rpm -q --whatprovides gimp-lang
no package provides gimp-lang
oct:~ # zypper se gimp-lang
Loading repository data...
Reading installed packages...

S | Name      | Summary                    | Type   
--+-----------+----------------------------+--------
  | gimp-lang | Languages for package gimp | package

Any other package providing gimp translation files is not installed as well.

It seems, that ZYPP ignored this recommends.