[RPG] Do humans start off with 1/3rd the languages of Other Races

In general in play they were ignored or just treated as an abstract language with no further comment.

As to where they came from, here's an answer from Gary Gygax on Dragonsfoot!

As D&D was being quantified and qualified by the publication of the supplemental rules booklets. I decided that Thieves' cant should not be the only secret language. Thus alignment languages come into play, the rational [sic] being they were akin to Hebrew for Jewish and Latin for Roman Catholic persons.

I have since regretted the addition, as the non-cleric user would have only a limited vocabulary, and little cound [sic] be conveyed or understoon [sic] by the use of an alignment language between non-clerical users.

If the DMs would have restricted the use of alignment languages--done mainly because I insisted on that as I should have--then the concept is vaible [sic]. In my view the secret societies of alignment would be pantheonic, known to the clerics of that belief system and special orders of laity only. The ordinary faithful would know only a few words, more or less for recognition.

In other words, it was supposed to be more like religious languages, but wasn't really well thought through. It disappeared in Second Edition and was not missed.

I have always read

You gain the selected language as a bonus language,

as meaning that you add the language to your list of known languages when you create your character (as a bonus language), not that you add the language to the list of bonus languages you can choose from to learn with a high intelligence modifier. The reason I believe this is that it says you "gain the language as a bonus language," not "add it to your available bonus language options" as other similar features do.

This is no different in my view from gaining a bonus language from a high intelligence modifier.

You apply your character's Intelligence modifier to:

• The number of bonus languages your character knows at the start of the game.

I interpret this as meaning that it is a bonus language because it is not a language you have learned through a rank in linguistics or from your racial features -- that is, it is a bonus on top of your standard known languages. Essentially, you have a list of languages to choose from, and when you gain one from high intelligence or a class features, it becomes a bonus language. They are bonus languages because they do not require ranks in linguistics to learn them (because a linguistics rank allows you to learn any language). When you become an oracle you gain that language as a bonus.

The text that follows,

At 5th level, pick an additional language to speak in combat and add it to your list of known languages,

indicates to me that the language added at 5th level is in addition to the bonus language gained at level 1.

Unfortunately, there is no real explanation for why the game writers used two completely different phrasings to mean the same thing, if you accept the premise in this answer as true, and, absent any developer or writer commentary on why such choices were made, I doubt we can make a determination on that.