What you said was irrelevant to the topic at hand. Regardless of how Spanish became dominant in Mexico, it in fact is and shows how a group of people with different regional languages are forced to use a common one when interacting on levels ecompassing all those disparate regions to fullfill basic needs.
No, it's directly relevant, even if you don't understand terms like diglossia or prestige. The situation in colonial societies cannot be compared to Germany or Poland.
Not all instances of language contact are the same. The imposition of Spanish or English over native populations was the direct result of massive violence, conquest and dispossession. It wasn't that they were forced to "interact with each other" (what the hell sort of outdated colonial fantasy world is that? do you think Europeans just waltzed in and unified a bunch of tribes or something?), it was that their societies were largely destroyed and they were forced to
interact with their conquerers, abusers, suppliers and jailers in the language of the conquerers, abusers, suppliers and jailers. The present situation also reflects the ongoing power imbalances of speakers of the native languages with speakers of the colonial language - the speakers of those languages are almost universally poor and uneducated and so the colonial language is essentially the only path to access basic services, any sort of education, etc.
None of this is applicable in Germany, Poland, Vietnam, etc, coherent societies that operate on their own internal languages. It's an enirely different language contact phenomenon, and you cannot confuse it with a colonial process. I don't get why so many people (anglophones again) don't really understand that there's no difficulty in not speaking the global lingua franca except when you're in certain fields. In a non-English-speaking society, you don't need English to repair cars or sell groceries to little old ladies or read the news on local TV or teach in primary schools or babysit someone's kids or join the infantry or do 1000 other things that don't involve long-distance phone-calls, reading foreign publications, managing a multinational company, etc. What the hell sort of future globalisation process do people imagine will happen that will lead English to replace these every day activities in, say, Italy or Portugal? There is no way in hell Swedes or Poles are ever going to need English in order to "fulfill basic needs" in their own countries.
There's an entire industry of translators and localisers who are perfectly capable of serving as a conduit between the products of major international languages and their own local one. That's not readily going to disappear. If thoroughly globalised Sweden can have a population which almost totally speaks English, with NO SIGN of Swedish being replaced as mother tongue or official language, then it's just not going to happen outside of that colonial setting.
The problem is you haven't defined your terms - you're talking about at least 4 linguistic concepts at once: language death, lingua francas (languages used beyond their native community, typically for trade), diglossia (the stable conexistance of two languages in one society, with one the language of learning and education, the other a personal language used with family and friends) and prestige languages (the language or variant a society values, sees as educated, sophisticated, etc). As a result you keep confusing entirely dissimilar phenomena and you're repeatedly giving the impression that you think if a language isn't used in international trade and diplomacy, it's going to disappear just like native ones in areas conquered by Europeans did, or are doing.
