You can add Russian to this extension by DIY config:

1. Open "about:addons" page and into "Option" page of this extension;
2. Click "Service Source API" in left navigation panel;
3. Select "Google" in top bar;
4. Find "include" section of JSON config part;
5. Add "ru" the Russian Lang code into array like others;
6. Done.

Step 4 that "include" section demo:
"include": ["auto", "en", "zh-cn", "ja", "ko", "es", "de", "ru"]

Or Remove "include" To Enable All Languages (Not Recommend).

More other usable languages code here:
https://github.com/wallenweel/weel-translate/blob/develop/src/api/languages.json