`hp-check -t` fails with non-English locale

[Stefan Löffler]

Output of `locale`:
LANG=de_DE.UTF-8
LANGUAGE=de_DE:en
LC_CTYPE="de_DE.UTF-8"
LC_NUMERIC=de_DE.UTF-8
LC_TIME=de_DE.UTF-8
LC_COLLATE="de_DE.UTF-8"
LC_MONETARY=de_DE.UTF-8
LC_MESSAGES="de_DE.UTF-8"
LC_PAPER=de_DE.UTF-8
LC_NAME=de_DE.UTF-8
LC_ADDRESS=de_DE.UTF-8
LC_TELEPHONE=de_DE.UTF-8
LC_MEASUREMENT=de_DE.UTF-8
LC_IDENTIFICATION=de_DE.UTF-8
LC_ALL=

Output of `lpstat -pXerox-Phaser-8560` [note the non-ASCII "ä" in "Mär"]:
printer Xerox-Phaser-8560 is idle. enabled since Mi 02 Mär 2016 18:35:32 CET

Output of `hp-check -t` (truncated):
Saving output in log file: /home/stefan/hp-check.log

HP Linux Imaging and Printing System (ver. 3.16.3)
Dependency/Version Check Utility ver. 15.1

...

Xerox-Phaser-8560
-----------------
Type: Unknown
Device URI: socket://130.113.54.232:9100
PPD: /etc/cups/ppd/Xerox-Phaser-8560.ppd
PPD Description: Xerox Phaser 8560DN
Printer status: printer Xerox-Phaser-8560 is idle. enabled since Mi 02 Mär 2016 18:35:32 CET
Traceback (most recent call last):
  File "/usr/bin/hp-check", line 856, in <module>
    num_errors, num_warns = core.validate(time_flag, is_quiet_mode)
  File "/usr/bin/hp-check", line 518, in validate
    log.info("Printer status: %s" % output.replace("\n", ""))
  File "/usr/share/hplip/base/logger.py", line 304, in info
    self.log_to_file("%s[%d]: info: :%s" % (self.module, self.pid, message))
  File "/usr/share/hplip/base/logger.py", line 211, in log_to_file
    self._log_file_f.write(message.replace('\x1b', ''))
UnicodeEncodeError: 'ascii' codec can't encode character u'\xe4' in position 96: ordinal not in range(128)

Output of `lsb_release -a`:
No LSB modules are available.
Distributor ID: Ubuntu
Description: Ubuntu 15.04
Release: 15.04
Codename: vivid

NB: Running `LANG=en lpstat -pXerox-Phaser-8560` gives the correct (i.e., English/ASCII) output. However, running `LANG=en hp-check -t` gives several "Gdk-WARNING **: locale not supported by C library" warnings and fails to detect cups (which is found correctly by `hp-check -t`).