Principal English Translation: 

by; among; in; to; between; in the company of; below; near; with; typically found as a locative suffix (on place names)

Lockhart’s Nahuatl as Written: 

(1) relational word. next to, below, among. often takes -ti- ligature after nouns, especially in proper names. -tlan nemi, to be the dependent of someone.
(2) relational word used only as suffix. place of. mainly in place names.
James Lockhart, Nahuatl as Written: Lessons in Older Written Nahuatl, with Copious Examples and Texts (Stanford: Stanford University Press and UCLA Latin American Studies, 2001), 237.

Attestations from sources in English: 

mictlān = (in) hell (lit., in the land of the dead)
Michel Launey, An Introduction to Classical Nahuatl, translated and adapted by Christopher MacKay (Cambridge: Cambridge University Press, 2011), 220.

achto tlahtohuani mochiuhca yn intlan teochichimeca = became the first ruler among the Teochichimeca
Codex Chimalpahin: Society and Politics in Mexico Tenochtitlan, Tlatelolco, Culhuacan, and Other Nahuatl Altepetl in Central Mexico; The Nahuatl and Spanish Annals and Accounts Collected and Recorded by don Domingo de San Antón Muñón Chimalpahin Quauhtlehuanitzin, eds. and transl. Arthur J. O. Anderson and Susan Schroeder (Norman: University of Oklahoma Press, 1997), vol. 2, 68–69.

When added to a noun ("x"), intending a placename, -tlan can mean the place of "x," in the midst of "x," or next to "x." In some translations, abundance is also indicated, e.g. place of abundant "x." This would really indicate the ending -tlah, but one will find intrusive "n's" added to such endings sometimes.

David Wright states: There are two morphemes that appear in colonial documents in Nahuatl with the written form -tlan. Both are often found at the end of place names. They have similar but not identical meanings, so it is important to distinguish between them.

1. The first of these two morphemes is -tla:n, with a long vowel, which is classified as a locative suffix. It can be translated as “among/in/near/place of/with.” It never appears with the ligature -ti-, nor can it be combined with possessive prefixes. Long vowels were rarely marked in colonial texts, so this suffix usually appears simply as -tlan. Sometimes the final ‘n’ is reduced to a diacritical mark over the preceding ‘a.’

2. The morpheme -tlan, with a short vowel, behaves differently from -tla:n. It has a locative sense; options for translation, depending on the context, are “among/between/in/in the company of/inside/near/next to/under/with.” Colonial grammarians classified this morpheme as a preposition, in spite of the fact that it follows nominal roots, because it belongs to a class of morphemes that express concepts of relation, much like prepositions do in romance languages. Since the late nineteenth century these morphemes have been called postpositions, which is a useful term, although some contemporary grammarians prefer to call them “relational suffixes,” “relational nouns,” “relational nominal nuclear clauses,” or “autolocatives.” These morphemes can be combined with the possessive prefixes no-, mo-, i:-, to-, amo-, im-/in-, and te- (e.g. notlan, “with me;” motlan, “with you;” i:tlan, “with him/her/it,” et cetera). We can distinguish the postposition -tlan from the locative suffix -tla:n because -tlan is combined with nominal roots using the -ti- ligature (e.g. Tenochtitlan). When -tlan appears as an element in a compound postposition (-cuatlan, -icxitlan, -nacaztlan, -tzintlan, and -tzontlan), or when it is combined with one of the possessive prefixes, it doesn’t use the -ti- ligature.

When -tla:n or -tlan follow a morpheme ending in the phoneme ‘l’, they become -la:n or -lan, due to a regular process of progressive assimilation (l + tl > ll), without altering their meanings.

There is another morpheme in colonial texts that resembles -tl:an and -tlan: the locative suffix -tlah (often written -tla, since the final “saltillo” or glottal stop was usually not represented). It does not appear with possessive prefixes, so it is not considered a postposition; neither is the -ti- ligature used with this suffix. It can be translated “where there is an abundance of (noun)”. The suffix -tlah changes to -lah when it follows a morpheme ending in ‘l’, due to the process of assimilation mentioned above.

Thus Zacatla:n means “among/in/near the grass,” Zacatitlan means “among/in/near/next to the grass,” and Zacatlah means “where there is an abundance of grass;” all contain the nominal root zaca, “grass.”
Contribution sent by David Write to the listserv Nahuat-L in February 2010 and kindly translated to English by him for this dictionary.

Nahuat-L, Feb. 27, 2010:
"Tlah or –tlan. The difference between the two locatives, -tlah and -tlan, is that -tlan is exclusively used to form a toponym. In most cases there is an assimilation of L+TL=LL: Tlaxcal-lan. Tol-lan. The use of –tlah is more generalized, a place, an agglomeration or a region. Te-tlah, a stony place. Cal-lah, an agglomeration. Mil-lah, a cultivated region.
This suffix could also be used with living beings: Tlaca-tlah, place with a lot of people. Lahun Ik 62 Baert Georges. Flanders Fields."
Nahuat-L listserv postings, February 27, 2010.

And, Michael McCafferty adds that -tlah can also refer to time: huehuetlah, ilamatlah.
Nahuat-L listserv postings, February 27, 2010.

Attestations from sources in Spanish: 

Parte de una discusión con Roberto Romero Gutiérrez y Tomás Amaya que vale la pena leer en todo, David Wright escribe:
1. El morfema ... -tla:n (así como su forma asimilada -la:n) tiene la vocal larga. Puede concebirse como un sufijo locativo. Algunas opciones para su traducción son "con/en/entre/junto a/lugar de", como mencioné. Nunca se presenta con la ligadura -ti-. Tampoco puede combinarse con los prefijos posesivos, a diferencia del segundo morfema, -tlan.
2. El morfema -tlan, con vocal corta, se comporta de una manera diferente al morfema anterior. ...[A]lgunas opciones de traducción, viables en distintos contextos, son "al lado de/cerca de/con/debajo de/en/en compañía de/en el interior de/entre/junto a". Los gramáticos novohispanos lo clasificaban como "preposición", a pesar de que suele colocarse después de las raíces nominales, porque pertenece a una clase de morfemas que expresan ideas de relación, de manera análoga a los preposiciones en el castellano. A partir de la segunda mitad del siglo XIX estos morfemas son llamados "posposiciones", término que me gusta, aunque hay quienes prefieren llamarlos "palabras de relación", "sustantivos de relación", "sufijos de relación" o "autolocativos". Estos morfemas pueden combinarse con los prefijos posesivos no-, mo-, i:-, to-, amo-, im-/in- y te- (por ejemplo, notlan, "conmigo"; motlan, "contigo"; i:tlan, "con él/ella/ello"; etcétera). Generalmente podemos identificar -tlan como un morfema distinto a -tla:n, porque -tlan se une a los sustantivos con la ligadura -ti- (por ejemplo, Tenochtitlan). Cuando -tlan aparece como un elemento de una posposición compuesta (-cuatlan, -icxitlan, -nacaztlan, -tzintlan, -tzontlan) o cuando se combina con prefijos posesivos, no se usa la ligadura -ti-.
En resumen, si vemos las letras -tlan después de un sustantivo, sin las letras -ti-, se trata del sufijo locativo -tla:n, con vocal larga, con alguno de los significados siguientes: "con/en/entre/junto a/lugar de". Si vemos la secuencia -tlan con -ti-, o después de un prefijo posesivo, o en las posposiciones compuestas, sabemos que es la posposición -tlan, con vocal corta, con alguno de estos significados: "al lado de/cerca de/con/debajo de/en/en compañía de/en el interior de/entre/junto a".
El morfema -tlah (y su variante asimilada -lah) ... es un sufijo locativo, no una posposición, porque nunca aparece con los prefijos posesivos, hasta donde llega mi (corta) experiencia. (Si alguien encontrara ejemplos de esta combinación en los textos coloniales, estaríamos obligados a reclasificar este morfema como posposición, de acuerdo con el criterio para clasificar que estamos usando.)
Así las cosas, los dos ejemplos ... que terminan en -tlan, Zacatlan y Tzihuactlan, tienen el sufijo locativo -tl:an, no la posposición -tlan, porque no contienen la ligadura -ti-: Zacatla:n, “en el pasto/junto al pasto/lugar del pasto”; Tzihuactla:n, “entre las plantas espinosas/junto a las plantas espinosas/lugar de las plantas espinosas”. (El tzihuactli es descrito en el Códice Florentino como una planta espinosa con forma de maguey.)
David Wright, Nahuat-L, 26 febrero 2010. Wright tambien cita a Joe Campbell: http://listserv.linguistlist.org/cgi-bin/wa?A0=nahuat-l&D=0&F=P

notlan = a mi lado
Miguel León-Portilla, "Los nombres de lugar en náhuatl," Estudios de Cultura Náhuatl 15 (1982), 39.

tetlan = al lado de algunos
Miguel León-Portilla, "Los nombres de lugar en náhuatl," Estudios de Cultura Náhuatl 15 (1982), 39.

notlan = conmigo
atlan = en el agua
itlantzinco = en él, con él
Rebecca Horn's notes from Nahuatl classes with James Lockhart. Card file being harvested by Stephanie Wood.