Yáʼátʼééh! This Navajo sentence has two words, and actually they are compound words. As far as I know, the first word is the subject, atsooʼ, with the prefix a- meaning that the noun is not a possessed form, then it would mean in English "a / the tongue / tongues" or perhaps "someone's tongue" (indefinite possessive), but I think there is already another conjugated noun for "someone's thing / object" in Navajo, but not sure of this. The important here is that this noun has a prefix indicating it does not have a possessor, and, I think because of that, it cannot be used as an object in a sentence where this noun is described with a possessor. Then, this use can be for general purpose, or to describe a tongue itself, without describing if it is forming a part of the whole body, or without describing from which body it is. It is a simple tongue (and other possibility, they would be "two tongues") that it is described without a possessor, neither more nor less. The following word is a verb, with a prefix, łi-, that is indicating it is an adjectival verb, then the verb itself is meaning "he / she / it is red" or "they (two) are red", since this conjugation can be singular or dual (duoplural), and being daalchííʼ / dałichííʼ the conjugation for plural, third person, for three or more.
Note: the root verb is -chiiʼ, with the low tone, and meaning "to be red". The verb conjugation with -chííʼ is denoting that the verb is in the neutral imperfective aspect (ni), and I think it can be used for a simple present, or continuous present tense in English, but the Navajo conjugation is not denoting a tense.
Note 2: I think this Navajo course would bring further information about how a noun is used in different conjugated forms.
In this exercise, the verb for the third person, singular and duoplural, is used with a noun without a possessor. But the same verb would be used for a conjugated noun, bitsooʼ, which is meaning in English "his / her / its tongue", or "their tongues", where the subject are two people, two animals, a person and an animal, a couple, two puppets or toys with tongues, etc.
I think the hard problem here is that in English, some sentences that use a possessive as, "my tongue", your tongue", "our tongues", or "their tongues" (for three or more possessors), can also use a same verb in the third person, is in the singular, and are in the plural. And my suspicion is that in Navajo it cannot be like that. :o
Then, it would be easy to translate a sentence that uses a third person possessor:
His / Her / Its tongue is red. = Bitsooʼ łichííʼ.
And with the duoplural, for two possessors:
Their tongues are red. = Bitsooʼ łichííʼ.
With the plural third person, for three or more possessors, things can be more confusing (I will try to show it easily):
Bitsooʼ (Their tongues) dałichííʼ (are red).
So, I think, it seems that the Navajo conjugated verbs for the third person are two, and the same as in English, with the difference that the number depends on the quantity of possessors, not on the quantity of nouns.
I insist that I cannot actually assure this, but it seems that the sentences with the first and second person possessors, in singular and plural, would present bigger problems.
I hope it helps. :)