Hi, I haven't seen the administrators for a while, so I'm welcoming you in their behalf. As you can see we write mainly in classical nahuatl (since it has been documented the most). But because we want to reach native speakers, most variants are welcome. Here you have a few resources, I hope you find them useful. --Marlon (discusión) 03:55 21 ic chicuēyi 2013 (UTC)
OK, I'll try to answer your question the best I can.
While it's true that most Spanish speaking countries separate decimals with a comma and use periods to separate sets of three digits, Mexico it's an exception. We use the same format Americans do. That's why you found contradictions. Probably the Portugal and Romania articles were written by people from Europe or South America (who use the format 123.456.789,12), but the Mexico article was written by Mexicans (who use the format 123,456,789.12).
I can´t assure that modern Nahua communities use the latter, but since the SEP uses it, and they produce Nahuatl language elementary textbooks, I think is appropriate to use a period as decimal mark.
As for the third option, it's possible to use just a space, as long as you do it with the decimals as well. (Like this: 123 456 789.987 65). There's an article in the Spanish wikipedia about the topic w:es:Separador_de_millares.--Marlon (discusión) 03:41 23 ic chicuēyi 2013 (UTC)