Imagine an English-speaking website, where everything is written in English. The author might or not speak another language, as well as the visitors, but all of them normally write in English when replying in that website, enabling them to communicate.
Then there comes the odd visitor which decides to write in his/her own language -- not English. And ... ta-da! Conversation ends up there. The other readers usually don't know what's in the message so they don't write anything else, just in case they go off-topic. So that's it. Discussion ruined in just one go, 90% of the times.
Occasionally it might be a different case, the non-English comment is absolutely ignored, then its author feels ignored and never writes again.
What should we do?
In this website, I have tried several approaches. First there wasn't any warning message, and people posted in whatever they liked. Usually if I posted in Spanish, they posted in Spanish too. If I posted in English, some of them posted in English and the rest of Spanish-speaking people replied in Spanish.
Then I added messages right before the comments form, but it didn't work: apparently no one reads the text before the comments form; they go straight to fill the Your name, Your email, Your url and Your comment fields. I could be insulting them and they would write comments anyway.
I also thought of sending the contents of the message to one of those nifty web services which detect in which language is a text written, to be able to ask them (again) to write in English, but it would not work either; they would just click OK and continue sending the message in Spanish anyway, as every user does when a pop up opens.
I even translated some comments from Spanish to English and let the authors know about that, asking them to write in English next time (which didn't happen), because even if I understand the comments, there might be more people which don't, and I didn't want them to feel excluded from the conversation.
The real truth
Some excuses were in the lines of saying
it's too much of an effort to write in English, or
I can't express my thoughts in English.
But since I never discuss metaphysical topics here and it's more about technical and programming stuff, I really doubt there's a lack of vocabulary or concepts which can be expressed in Spanish but not in English. In fact, it's usually the opposite; I find very complicated to write about some tech stuff in Spanish because I wouldn't know how to translate certain terms.
Let me guess... Is it laziness? Are Spanish speakers lazier than anybody else? No, it can't be that. If they were truly lazy, they would even avoid commenting - that would be an effort! Not only articulating a thought but also pressing the keys in the right sequence in order to write it down... yikes! And I have seen also Italians, French, German, Brazilian and a loooooooong etc doing this. So it ought to be a different reason.
And I have the feeling that it's something more profound which has something to do with the shame that makes them apologise for their bad English whenever they feel brave enough to write in English. There's some kind of inferiority complex, and as such, it acts as a barrier when interacting with the rest of us.
Listen up, there's nothing to be afraid of. No one was born instantly knowing any language and it's better to half communicate than not to communicate at all. So stop writing in Spanish when a page is in English and join the club of the people which can communicate across the globe no matter where they are from :-)