Thieves' Cant isn't a written language, thus there would be nothing to understand via a spell.
Nowhere in the quote you've pulled (or the PHB) is thieves' cant ever described as a written language. This is because thieves' cant is both verbal and physical communication. Some word substitution (1 to 1) is used, but it is largely based on metaphor and contextual meaning and a big part of this is the hand symbols used when speaking. D&D's basis for thieves' cant is both historical and a trope.
The symbols mentioned are more like pictographic signs than words.
As such they are not translated, but identified, similar to how we use symbols such as the biohazard sign and nuclear sign to signify specific danger or how the symbols on a crosswalk signify when to wait and when to go. The closest living example of this I can highlight would be Hobo symbols that survive and are still in use today in the US. Different symbols would mean different things to different groups and insider knowledge for understanding thieves' cant symbols would be a must.
- I'm sure I've seen it before but cant find it, but what counts as an intelligent creature?
Any creature with an intelligence score.
The Intelligence section of the Ability Scores chapter clarifies that:
Intelligence determines how well your character learns and reasons. This ability is important for wizards because it affects their spellcasting ability in many ways. Creatures of animal-level instinct have Intelligence scores of 1 or 2. Any creature capable of understanding speech has a score of at least 3. A character with an Intelligence score of 0 is comatose. Some creatures do not possess an Intelligence score. Their modifier is +0 for any Intelligence-based skills or checks.
I bolded the part that should interest you more. That section also gives a a couple of exceptions for creatures with no intelligence score:
Animals have Intelligence scores of 1 or 2 (no creature with an Intelligence score of 3 or higher can be an animal).
Oozes do not have an Intelligence score, and as such they have immunity to all mind-affecting effects (charms, compulsions, phantasms, patterns, and morale effects). An ooze with an Intelligence score loses this trait.
Regular plants, such as one finds growing in gardens and fields, lack Intelligence, Wisdom, and Charisma scores; even though plants are alive, they are objects, not creatures.
Vermin do not have an Intelligence score, and as such they have immunity to all mind-affecting effects (charms, compulsions, morale effects, patterns, and phantasms). Mindless creatures have no feats or skills. A vermin-like creature with an Intelligence score is usually either an animal or a magical beast, depending on its other abilities.
And the int score table gives us three examples of creatures that do not have an Intelligence score (their Intelligence is "-"):
Zombie, golem, ochre jelly
The Animal subtype also confirms that a creature with 3 or more intelligence is not an animal. But be careful to not confuse that with animal companions with high int scores.
Now, we also know that animals can be trained, and even learn how to understand speech (with ranks on Linguistics). But most animals cannot speak unless you get them a Circlet of Speaking.
Tongues does not enable the subject to speak with creatures who don't speak.
This clause on the spell description is the only thing that prevents the spell from being used to understand animals, so we have to use Speak with Animals instead.
- Can I cast the spell on myself and speak with every summoned creature (provided it counts as intelligent)?
Yes.
You are satisfying all the requeriments for the spell to work.
- If that does not work, can I cast this spell on my summons so they can understand me, as the spell doesn't say the target must be intelligent?
Yes.
However, the spell says your target can understand speech, but does not say they acquire the necessary intelligence score to follow and rationalize whatever is told him.
They will be limited by their own intelligence score. So an animal will understand "danger", "hunger" or "help", but will not understand "what direction did the bandit go?", because that is probably too much for their limited intelligence.
Score Examples Description
-: Zombie, golem, ochre jelly
0: Comatose
1: Carrion crawler, purple worm, camel Lives by the most basic instincts, not capable of logic or reason
2-3: Tiger, hydra, dog, horse Animal-level intelligence, acts mostly on instinct but can be trained
4–5: Otyugh, griffon, displacer beast Can speak but is apt to react instinctively and impulsively, sometimes resorts to charades to express thoughts
6–7: Troll, hell hound, ogre, yrthak Dull-witted or slow, often misuses and mispronounces words
8–9: Troglodyte, centaur, gnoll Has trouble following trains of thought, forgets most unimportant things
10–11: Human, bugbear, wight, night hag Knows what they need to know to get by
Best Answer
Yes, its possible to learn and speak druidic without being a druid.
The Faiths & Philosophies player companion introduced the Druidic Decoder feat, that allows a character to decode and even learn druidic without taking levels of druid.
This means that the language isn't magically protected from means to learn and speak it by non-druids, they merely don't share it's secrets around, nor publish any dictionaries that can be found on the local library.
Keep in mind that you do not choose languages when you cast Tongues, you simply grant the ability to speak any language of any intelligent creatures. The spell doesn't grant the knowledge of that language, it simply makes communication possible. Similarly, the spell Comprehend Languages will allow you to understand what is written on a druidic text, but not give you the knowledge to be able to translate it to common.