The editing help for comments currently lists a few shorthand links which can be used to link to other sites:
[se]– link to the given site. Link text is the site name.
[something.se]– link to something.stackexchange.com, if that site exists. Link text is the site name. Use
[ubuntu.se]for Ask Ubuntu.
Stack Overflow em Português is the only non-English Stack Overflow site listed, but
[ru.so] work as well. Can these be added? Furthermore, if
[ubuntu.se] is listed as an exception,
[mathoverflow.se] should be added as well. Not many people know that's a valid alias.
Also, it makes sense to rearrange the bullets a bit. The two discussed above should be next to each other, since they serve the same purpose: links to another Q&A site. They should probably be next to
[chat], while the one about
[answer] (both link to Help Center pages) should be next to the one starting with