%%GLOBAL_HeadFonts%%
is used to fetch the base theme's fonts. Since you have no control over any %%GLOBAL.something%%
and %%LNG.something%%
its better to call your fonts just under this line. You can upload your fonts by creating a fonts directory (under template) then call those fonts either within your Style.css or externally using %%ASSET.fonts/myfont.wof%%
Use this https://developer.bigcommerce.com/themes/panels/html_head to understand BigCommerce HTML.head architecture.