Bug fix: add PYTHONPATH for contrib/check_i18n.py

for plugin i18n.py, it will import the base i18n.py
The i18n.py may from install directory of source directory.

Signed-off-by: ShaoHe Feng <shaohef@linux.vnet.ibm.com>
Signed-off-by: Mark Wu <wudxw@linux.vnet.ibm.com>
Signed-off-by: Zhou Zheng Sheng <zhshzhou@linux.vnet.ibm.com>