What seems to be lost in the translation is the culture the immigrants bring with them and the attitude they have toward integration within the existing culture to which they've entered.
There are two attitudes here. Immigrants who wish to be 'Americans' and integrate themselves into the American culture and immigrants who are only here for the money.
Your Czech friend wanted to become an American, so he learned English and became a citizen. The Mexican illegal aliens do not wish to become Americans and thus, do not learn the language, or only learn enough to get by and make no attempt to become citizens. They make no attempt to integrate into American society as citizens and instead, bring their cultural 'heritage' here with them.
Lastly, you didn't answer my quetion. You mentioned that change is inevitable. So, once again, I ask; Has the influx of large numbers of Mexican illegal aliens changed L.A. for better or for worse?
I answered your question.