Language Footprints and Individualism
Lauredhel mentions the idea of a "language footprint." Patterned after the common idea of an ecological footprint, your language footprint is the extent to which your activities negatively impact the survival of other languages.
Raising awareness of the problem of language extinction, in which a few global languages drive out other languages spoken by smaller and less powerful groups*. But I think the "footprint" metaphor tends to frame the problem in too individualistic a way. The description on the language footprint site lends itself easily to being thought of as a matter of individual choices of language, a la a rude tourist asking the waitstaff for "water" rather than "agua" or "yaku." And indeed, the comment section of the above-linked Hoyden About Town thread is full of people proudly professing that they study a phrasebook before travelling internationally.
The issue here is not that learning the local language before traveling is unimportant**. It's that it addresses only a small aspect of the problem of language extinction. Language extinction is fundamentally a structural problem. English is not encroaching on other languages because Anglophones are ruder than speakers of Pitjantjatjara or Saami. English is encroaching on other languages because our world is becoming more and more globalized and the global political-economic system is dominated by English-speaking countries.
Ecological footprint calculations can break down this sort of structural responsibility and apportion a share of the blame to individuals to a limited degree, because it's possible to distinguish differential environmental impacts of different lifestyle and consumer habits -- driving a Prius vs an SUV, being a vegan vs a meat-and-potatoes person, etc. (I forget where I found this first, but it's interesting to note in connection with the structural factors in environmental impacts that even Buddhist monks in the USA have twice the world average ecological footprint.) But with respect to language dominance, the negative structural effects on smaller languages are a result of the system as a whole (i.e. the level of global integration and dominance of Anglophone countries), making it next to impossible to talk about more or less "language-friendly" practices beyond the small potatoes of learning the languages of your tourist destinations. Certainly one could engage in activism aimed at changing these structures (either at a global level or in more locally-focused projects), but factoring that in as an element of your individual language footprint seems to warp the metaphor beyond utility.
*This can be a complexly multi-scalar process. If I recall my freshman-year class on Peru correctly, English is encroaching on Spanish at the global level, while within Latin America Spanish is encroaching on indigenous languages such as Quechua, and at the same time Quechua is actually encroaching on less-popular indigenous Andean languages as the region's Indians consolidate a new form of native identity in the face of globalization.
**Although it is interesting to me the different motivations involved. I've only traveled once to a non-English-speaking area, and while I did study my German phrasebook en route to Bonn, it was less out of a desire to be conscientious about my impact on German speakers or a desire to get a real experience of local culture, than it was because I was afraid I might get hurt and not be able to tell people "Rufen Sie die Krankenvagen!"