I think they could add the onclick to any page. not 100% sure though, didn't test it.
<a class="readon" onclick="s5_custom_7_open()">Check Availability</a>
It would still need the contact widget published to the page in Custom 7 though too.
As for the language, there isn't an easy way to translate it.
They would have to edit the /s5_quick_contact/pikaday.js file and look for the lines in there to change; for example, around line 241:
previousMonth : 'Previous Month',
nextMonth : 'Next Month',
months : ['January','February','March','April','May','June','July','August','September','October','November','December'],
weekdays : ['Sunday','Monday','Tuesday','Wednesday','Thursday','Friday','Saturday'],
weekdaysShort : ['Sun','Mon','Tue','Wed','Thu','Fri','Sat']