[Back-end]: Minor issues connected with localization

Preconditions:
1. Billing is available on DataLab Web UI

Steps to reproduce:
1. Go to 'Billing report' page

2. Click 'Export' button

Actual result:
1. Available reporting period is  not localized according to your browser setting

2. Currency symbol is is localized according to your browser setting

Expected result:

1. Available reporting period is localized according to your browser setting

2. Currency symbol is is not localized according to your browser setting. Instead of 'USD' should be '$' (before/after sum. it depends on your browser language)