add Thai language file.

However, in Thai, we do not use the first 3 letters for month and day abbreviations.
Instead, we have predefined abbreviations for months and days, like so:

```php
// month abbreviations
'jan' => 'ม.ค.',
'feb' => 'ก.พ.',
'mar' => 'มี.ค.',
'apr' => 'เม.ย.',
'may' => 'พ.ค.',
'jun' => 'มิ.ย.',
'jul' => 'ก.ค.',
'aug' => 'ส.ค.',
'sep' => 'ก.ย.',
'oct' => 'ต.ค.',
'nov' => 'พ.ย.',
'dec' => 'ธ.ค.',

// day abbr.
'mon' => 'จ.',
'tue' => 'อ.',
'wed' => 'พ.',
'thu' => 'พฤ.',
'fri' => 'ศ.',
'sat' => 'ส.',
'sun' => 'อา.',

```

And Thai uses Buddhist calender which equivalent to the Western Year + 543.
For example, 2014 (Western Year) = 2557 (Buddhist Year)