Look at the requirements: a small clay model of a ziggurat. I see what they're doing. It's a bible reference, isn't it!

The Tower of Babel.

I thought so.

That’s where God stopped everyone from using the same language, so that they would spread over and populate the earth, and stop building the Tower of Babel.

If I cast this, and then cast Major Illusion, and then had the illusion speak, would any creature who hears it be able to understand what it says?

For example, my Bard does not speak Orc, and casts tongues on himself, then casts an illusion of an orc god. If the illusion spoke, would orcs who only understand orc be able to understand it? 

Major Image doesn't give you any special ability to have the illusion speak languages you don't understand. Tongues doesn't let you know any languages, just understand them when they're spoken. So, no, what you described won't work.

However: There’s no saying you couldn’t cast Tongues and then use Disguise Self or wear the Major Image as something like a skin suit.

Quote from Grygus_Triss >>

If I cast this, and then cast Major Illusion, and then had the illusion speak, would any creature who hears it be able to understand what it says?

For example, my Bard does not speak Orc, and casts tongues on himself, then casts an illusion of an orc god. If the illusion spoke, would orcs who only understand orc be able to understand it? 

 Major image allows “Similarly, you can cause the illusion to make different sounds at different times, even making it carry on a conversation, for example.”. But doesn’t say how you do that.  Tongues allows anyone who hear you (if it is cast on you) that understands at least one language to understand you.

so it is a gray area if you could make the image speak and if anyone who understands a language could understand it.  Perhaps if you made it copy what you were saying your DM might allow it.

also since objects can pass through the image you could stand where the image is as long as it is bigger than you and it would appear as if the image is speaking.

however any creature who speaks at least one language would understand you not just ones that speak orc.

Moreover, when the target speaks, any creature that knows at least one language and can hear the target understands what it says.

question - when they understand you do they hear you in thier primary language or hear you speaking your language just understand what you are saying?

Quote from lizmorn >>

Moreover, when the target speaks, any creature that knows at least one language and can hear the target understands what it says.

question - when they understand you do they hear you in thier primary language or hear you speaking your language just understand what you are saying?

 I think a bit of both. They'd magically understand you first, and their own brains would comprehend that understanding as their primary language.
That's how I've always ruled it and it's how my dm's have played with it. It's a good bit of flavor and makes it a lot easier on how npc's and pc's would react to hearing somebody use it

I have been searching everywhere for a Spell to speak another language. I have been wanting to speak fluently like a native speaker and I would do anything to speak that way. Can this spell help me?

I'm disappointed this spell does not have a somatic component and that component does not involve licking the person on whom you are casting the spell.

Quote from EVEL_Bug >>

I'm disappointed this spell does not have a somatic component and that component does not involve licking the person on whom you are casting the spell.

 That sounds like a Claim spell, but it would only work on objects.

Quote from BaleCastle >>

I have been searching everywhere for a Spell to speak another language. I have been wanting to speak fluently like a native speaker and I would do anything to speak that way. Can this spell help me?

Absolutely, for one hour at a time! 3rd Lvl spell slots can be kind of scarce at low levels, in which case you could try asking your DM about a common or uncommon magic item that would cast this spell for you. 

Alternatively, the Eldritch Adept feat would allow you to "read all writing" (but not write!) and at 1st Lvl, choosing The Great Old One as your Warlock patron grants Awakened Mind, which lets you telepathically speak to any creature you can see within 30 ft (but not "with" them!). By multiclassing 1 level as a GOO Warlock, you could at least speak to someone & ask for a written response? ("What's the cost on paper & ink again?") Pair this with Comprehend Languages & you're in business! 

So a character with 1 level of GOO Warlock who knows Comprehend Languages can cast that to understand anyone speaking, then answer them telepathically. Cost: One Lvl 1 spell slot. Duration: 1 hr. 

Edit: Also, there's the Helm of Comprehend Languages, an Uncommon magic item that might go for 101-500 gold pieces, at the DM's discretion. Doesn't require attunement, so a character with 1 Lvl of GOO Warlock could wear this & converse freely at all times.

If Tongues isn't available & you're not that kind of Eldritch, maybe you can obtain access to Speak With Animals & get some inside dish from their mount, or Speak With Plants to find out which way someone went, or have the person poisoned & then cast Speak With Dead on them. 

(Just trying to cover all the bases here)

I wish this was available to Artificers, because then I could do the Hitchhiker's Guide "put this fish in your ear" gag

If a creature fails its saving throw while casting contact other plane or against a symbol spell and becomes insane, could you cast tongues on the creature to negate some of the effects of the insanity? Per insanity:

While insane, you can't take actions, can't understand what other creatures say, can't read, and speak only in gibberish.

 Would tongues suppress the "understand what other creatures say" portion? Could one understand the gibberish that an insane creature speaks? 

What would happen?

*disappointed noises because this isn't a ritual spell.

Short question, if I cast "Tongues" on myself and use the feat "Observer", do I understand the spoken words - even if I don´t hear them?

I would say no because the rules for "Tongues" say "understand any spoken language it hears" so the ability to hear is important to the spell. It is like the aforementioned  "Fish from Hitchhikers Guide to the Galaxy".

In the same spirit, Tongues would not allow someone to understand Drow Sign Language

 Would tongues suppress the "understand what other creatures say" portion? Could one understand the gibberish that an insane creature speaks? 

What would happen?

The insane creature can't take actions and their movement is erratic and uncontrolled. Whether or not they can understand what someone is saying wouldn't have any effect on their behavior.

And no one could understand the gibberish the insane creature speaks because gibberish by definition contains no meaning. There's nothing there to understand.

Good news is the insanity effect of both these spells ends fairly quickly, so there's no need to waste a spell slot anyways.