Bugfix: Set locale with force in BackendModel::getURLForBlock()

## Problem

When having a website in 'nl', 'fr' and 'de', but when 'de' is not yet activated to be shown in the frontend.
The Backend triggers an error when you are on a module its edit.php.
Error triggered by:
```
$url = BackendModel::getURLForBlock($this->URL->getModule(), 'detail');
```
Error-message: ```Invalid language (de).```

## Solution

We use the build in "force to set locale" setting and the problem is solved.
In ```BackendModel::getURLForBlock()```, we replaced
```
FrontendLanguage::setLocale($language);
```
by
```
FrontendLanguage::setLocale($language, true);
```