Barring a few key exceptions, Classical Nahuatl vocabulary for Old World countries were usually written according to Spanish convention.
Chimalpahin's Nahuatl-language annals writes "China" as
China, "Emperor of Japan" as
Emperador Jabon (or
Japon or
Jabun), France as
Francia, Italy as
Italia, and so on. There are some exceptions, like one 1740 document that writes "Florencia" (the Italian city of Florence) as
Tlorencion, but in general Spanish orthography is respected.
Nahuatl didn't actually have phonemes like /f/ or /r/, but this didn't matter because Nahuatl speakers educated enough to know about the existence of Japan and France were bound to speak some level of Spanish anyways.
The main exception to just using whatever Spanish word there is is the early Nahuatl word for "Spain,"
Caxtillān. This is the Spanish word "Castilla" made to fit Nahuatl morphology;
Caxtillān means "Place of
Caxtil," using the —(
t)
lān suffix you also find in native Nahuatl place names like
Tlaxcallān (Tlaxcala) and
Cuauhtēmallān (Guatemala). In the sixteenth century,
caxtil was often used as the word for European chickens, the most obvious thing that came from
Caxtillān (which could now be retro-analyzed as "Place of Chickens"). But it's the exception that proves the rule, really, and
Caxtillān increasingly yields to
España as the colonial period progresses.
In a TL where Nahuatl becomes the official language of Mexico following a protracted period of Spanish rule, I don't think this situation would change barring a extremist nationalist movement trying to "purify" the language by forcibly purging the language of Castillian vocabulary—and even then it would be tough and ugly, like trying to purge the English language of all Romance loanwords.
In a TL where Nahuatl-speaking states survive, you'd probably still see Spanish place names being adopted, just as many African languages use whatever English names there are for European countries. But with less direct colonial influence, Spanish phonology is less likely to be emulated, and at some point a spelling reform to make the
orthography shallower could result in names like (given what we know of how colonial Nahuas pronounced and occasionally wrote Spanish loanwords):
- France: Palānciah or Tlānciah (from Sp. Francia)
- England: Incalatēlah or Calatēlah (from Sp. Inglaterra)
- Portugal: Polotocāl or Potocāl (from Sp. Portugal)
- Alternately, a Nahuatl speaker might also analyze Spanish Portugal as Poto-calli, with the Nahuatl suffix —calli meaning "house." Then Portugal could be understood as the "House of Poto," just as Castilla was analyzed as "Place of Caxtil," and a new word for "Poto" could be found to refer to whatever is characteristic of Portugal
- Constantinople: Contantinōplah or Coztantinōplah (from Sp. Constantinopla)
- Jerusalem: Xelozallān (from Sp. Jerusalén)
- Again, an interpretation of Spanish Jerusalén as Xelozal-lān, the "Place of Xelozal." It's up to the Nahuatl speaker/TL writer to decide what Xelozal might mean.