passing of `test_to_datetime_with_non_exact` depends on OS language.
The abbreviations of the names of months in `format_regex` are language specific. Thus the test fails on non-english systems: 

`format_regex` on this machine is

 `re.compile('(?P<d>3[0-1]|[1-2]\\d|0[1-9]|[1-9]| [1-9])(?P<b>jan|feb|m\\är|apr|mai|jun|jul|aug|sep|okt|nov|dez)(?P<y>\\d\\d)', re.IGNORECASE)`

```
ERROR: test_to_datetime_with_non_exact (pandas.tseries.tests.test_timeseries.TimeConversionFormats)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/ch/repo/pandas/.tox/py34/lib/python3.4/site-packages/pandas/tseries/tools.py", line 363, in _convert_listlike
    values, tz = tslib.datetime_to_datetime64(arg)
  File "tslib.pyx", line 1451, in pandas.tslib.datetime_to_datetime64 (pandas/tslib.c:22987)
TypeError: Unrecognized value type: <class 'str'>

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/Users/ch/repo/pandas/.tox/py34/lib/python3.4/site-packages/pandas/tseries/tests/test_timeseries.py", line 4335, in test_to_datetime_with_non_exact
    result = to_datetime(s,format='%d%b%y',exact=False)
  File "/Users/ch/repo/pandas/.tox/py34/lib/python3.4/site-packages/pandas/util/decorators.py", line 89, in wrapper
    return func(*args, **kwargs)
  File "/Users/ch/repo/pandas/.tox/py34/lib/python3.4/site-packages/pandas/tseries/tools.py", line 267, in to_datetime
    unit=unit, infer_datetime_format=infer_datetime_format)
  File "/Users/ch/repo/pandas/.tox/py34/lib/python3.4/site-packages/pandas/tseries/tools.py", line 373, in _to_datetime
    values = _convert_listlike(arg.values, False, format)
  File "/Users/ch/repo/pandas/.tox/py34/lib/python3.4/site-packages/pandas/tseries/tools.py", line 366, in _convert_listlike
    raise e
  File "/Users/ch/repo/pandas/.tox/py34/lib/python3.4/site-packages/pandas/tseries/tools.py", line 336, in _convert_listlike
    arg, format, exact=exact, errors=errors)
  File "tslib.pyx", line 3018, in pandas.tslib.array_strptime (pandas/tslib.c:44557)
ValueError: time data '19MAY11' does not match format '%d%b%y' (search)
```
