When I use Google Fonts Nirvana 0.9.9 is not employing the Font Weights. I’ve specified multiple weights for the General Font (i.e. Open+Sans:300,500,700) and in the specific instances (i.e. Open+Sans:500).
If it is a matter of something I am doing wrong, please let me know.
Did you try this with a font that’s not already included in the theme?
Technically, forcing a font weight via the Google font field will not work as the other font weights are already included in the font that’s in the theme, and browsers will use the weight specified by the CSS (usually 300/normal).