From my knowledge there is no such language. Angels have this ability, but the languages they speak (Celestian and a few others) are just ordinary languages with no special properties. It's a supernatural ability of theirs which gives them a cheat mode:
In both games that ability takes the following form:
All angels can speak with any creature that has a language, as though using a tongues spell (caster level equal to angel’s Hit Dice). This ability is always active.
The Tongues spell this ability refers to is also identical in both games (D&D 3.x Tongues and Pathfinder Tongues):
This spell grants the creature touched the ability to speak and understand the language of any intelligent creature (snip)
So: there's no cheat-mode language for mortals to learn. They just need to have Tongues. If they're using that spell, they follow its rules and can understand and speak other languages.
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.
Best Answer
I'll split my answer into two:
Are you speaking the language?
From the spell description:
As it is written, I understand it as: You are able to hear the language and people hearing you understand as if you were speaking a language that they know. It is not clear, though, which language is being heard, mainly in the case where the listener knows more than one language.
So, in your example, let's say there is this Dwarf City where everyone speaks Dwarven (and only dwarven, anyone not speaking dwarven is seen as a barbarian and should be killed) and you want to blend in. Nothing on the rules state that the language they will be hearing would be Dwarven instead of simply Common.
Is it obvious that you are using a spell?
The casting takes one action and the duration is one hour. You don't have to do anything in order to maintain the spell, so it shouldn't be obvious that you are using a spell, at least nothing in the text describes it.
Answer
I think I've been using this phrase too much lately, but... ask your DM. If you are the DM, decide on what makes more sense. RAW, it doesn't seem to explicitly prevent or allow what you want to do.
Tongues is more of a flavor spell (unless your DM wants you to use it and creates a scenario where you have to communicate with something or someone that doesn't speak any common language with anyone in the party) so actually giving it an interesting utility should be fine and the player using it could feel motivated.