Now, in a lot of countries and regions, it makes sense to only allow the official language. However, there is no official language in the United States of America. Therefore, the line has to be drawn somewhere. While there is technically no official language, all legal documents in the USA are written in English, so I feel that English makes sense to be the only language tournament-legal in the USA.
Imagine if everyone was able to use cards of their language, whether primary, secondary, tertiary, or any. We'd have people playing English, French, Polish, Spanish, Portuguese, Italian, Japanese, Korean, and German cards at tournaments. Now, this would involve numerous translators that know numerous languages, either to translate what the other person is saying (if they don't know the other player's language), or to verify what is said in order to lessen cheating. You could have a Hispanophone playing a Francophone, an Italophone playing a Lusophone, all at a small-scale Cities-type event. While this would not be an issue at Worlds, one can only imagine the havoc caused at small events.
Of course, there's always the issue that not all Americans speak English. I know a number of people that only speak Spanish, as well as a few that only speak Italian. There must be some way to accommodate them. While the 10% rule might help in this case, people that don't know any English would still not be able to comfortably play. So, I think we have a bigger issue at stake here; players are being turned away because of their language.
Really, there's no good solution to this issue, either. Off the top of my head, the only thing I can think about is language-specific tournaments, though this would definitely mess with rankings.
Imagine if everyone was able to use cards of their language, whether primary, secondary, tertiary, or any. We'd have people playing English, French, Polish, Spanish, Portuguese, Italian, Japanese, Korean, and German cards at tournaments. Now, this would involve numerous translators that know numerous languages, either to translate what the other person is saying (if they don't know the other player's language), or to verify what is said in order to lessen cheating. You could have a Hispanophone playing a Francophone, an Italophone playing a Lusophone, all at a small-scale Cities-type event. While this would not be an issue at Worlds, one can only imagine the havoc caused at small events.
Of course, there's always the issue that not all Americans speak English. I know a number of people that only speak Spanish, as well as a few that only speak Italian. There must be some way to accommodate them. While the 10% rule might help in this case, people that don't know any English would still not be able to comfortably play. So, I think we have a bigger issue at stake here; players are being turned away because of their language.
Really, there's no good solution to this issue, either. Off the top of my head, the only thing I can think about is language-specific tournaments, though this would definitely mess with rankings.