Players may use otherwise-legal non-English and/or misprinted cards provided they are not using them to create an advantage by using misleading text or pictures. Official promotional textless spells are allowed in DCI-sanctioned Magic tournaments in which they would otherwise be legal. Artistic modifications are acceptable, provided that the modifications do not make the card unrecognizable or contain substantial strategic advice.
The Head Judge is the final authority on acceptable cards for a tournament.
The last sentence is probably the most important. If the cards aren't in English and the art is completely obscured, there is a good chance the judge might say this card is "unrecognizable" in an English-speaking area. The means by which the cards were altered is irrelevant insofar as they do not cause the card to be thinner or thicker than a standard Magic card. Once the substance of the card is significantly changed in that way, however, that card is marked and can't be played. Again, however, it's ultimately up to the judge.
As is usually the case when judge discretion is involved, when in doubt, ask the head judge before the tournament starts or just don't do it at all.
