fix getPreference function (#527)

the getPreference would only check the URL parameter and not the actual
preferences for the key "application_language". This resulted in an
always-English frontend independent of the preferences set in the prefs
file.

The issue was reported by @obertsalome and can be reproduced with the
current Pintos data by simply starting the Edirom without any URL
parameter. This will show the frontend GUI elements in English although
the "application_language" is set to "de".


## Types of changes
- Bug fix (non-breaking change which fixes an issue)
- Improvement

## Overview
- I have performed a self-review of my code, according to the [style
guide](https://github.com/Edirom/Edirom-Online/blob/develop/STYLE-GUIDE.md)
- I have read the
[CONTRIBUTING](https://github.com/Edirom/Edirom-Online/blob/develop/CONTRIBUTING.md)
document.
- All new and existing tests passed.