I don't think it is ungrammatical. However, "interested" is more often followed by "in" than "to" with action verbs. In other words, the gerund is more common than the infinitive when used with "to be interested."
You will hear, for example, "I'm interested to hear what you think," or "I'm interested to see how it looks," and so on. Here, however, it is much more common to say "I'm interested IN playING something."
I don't understand how interesar is conjugated. Is it the same as gustar? So far I've only seen it as interesa. It isn't always interesa, is it?
Interesar is conjugated as a normal verb, but what's changing things here is the the object pronoun me.
Without the me it would be the he/she/it/you (usted) conjugation interesa - it interests. So what this sentence is really saying in literal translation:
It interests me to play at the football
That's not particularly good English grammar, so it gets changed to:
I'm interested in playing football.
It's important to remember that quite often you can't make literal Spanish to English translations.
Angielr1, to answer your other question, yes. It's like gustar. So are encantar and importar.
Me encanta jugar al fútbol. Me importa jugar al fútbol.
These verbs are used most often in 3rd singular and plural: gusta, gustan.
When speaking of playing a game or sport, you are generally correct. While it is considered okay to use "jugar" as a transitive verb with sports in some parts of Latin America, that only applies regionally and not generally in Spanish. That said, you can use "jugar" as a transitive verb in other situations and then the meaning is closer to betting or playing a hand/card, etc. Finally, you can also play a match/game (jugar un partido).
There's more info here, from RAE.
So, do they not play what Americans consider "football" in Spanish-speaking countries? I cannot seem to find the Spanish translation for American "football".
I thought it was fútbol de americano (is what i think came up with other lessons of duolingo)
Can someone please explain to me why el is used sometimes and othertimes al? I read through the comments and I wasn't able to find clarity..
Al is used instead of putting a el. When a phrase requires a such as jugar a and a singular masculine noun with el comes next, you put them together to make al. Jugar al tenis = to play tennis. When the "a" is not required you would see el. El baloncesto = basketball