My problem is how to add new language like thailand language in liferay 7.
When I inspect the existing flag I see that the image is not png but it is svg, but
Assuming that you have followed all the steps in this Liferay blog, adding the flag in Liferay 7 requires:
Create a new theme, or modify existing theme to :
images\lexicon\flags-xx-YY.svg (where xx-YY is your locale, case-sensitive)open the file icons.svg in the same folder and add your flag SVG elements as a symbol among the other symbol elements:
Deploy, or re-deploy, the theme and choose it as the active theme for the pages you want.
Clear the browser cache if needed.