First, I have a number of guides and FAQs that answer some of your questions:

https://wiki.spiralknights.com/User:Jdavis

1. On an alternate knight, I played for about 49 hours before reaching the 5-star Hall of Heroes (with 4-star equipment). If I had continued playing that knight, then I would have had to farm for many more hours, to get the 5-star items needed to clear that hurdle. Anyway, this story gives you some idea.

2. Your loadout is a matter of personal taste and style. You haven't told us whether you favor swords, guns, bombs, or a mix. The two most popular missions for farming resources are King of Ashes (crowns) and Dreams and Nightmares (Radiant Fire Crystals). So you might want to equip for those. For KoA, popular weapons include Blitz Needle and Combuster. For DaN the options are more diverse. For armor see my Armor Guide and for shield see my shield guide.

3. Guilds are mostly the same as in 2011. Some guilds are active in PvE (you can find teammates for missions, etc.). Some guilds are into PvP and require you to be. Some guilds contain people who say irritating things. Join a guild, see whether you like it, leave if not, join another guild, etc. There is no reason to delay starting the process.

4. I don't have numbers on the current player base. Sometimes we have multiple really active instances of Haven, and sometimes we don't. The game population has been declining since 2011. Many of the knights you see are old, or alternates for old knights. But there are new players around too, and the game is great fun, so don't be discouraged.

5. The English-speaking population is probably bigger than the other languages' populations. For example, Grey Havens ended support for German and French in 2016.

6. To get fire crystals, see my FAQ linked above. It tells you general principles and includes links to data sets on specific missions and Clockworks levels.

7. It depends on what you mean by "nice". People are nice in that they don't yell at you. People will even help you with missions sometimes (especially within a guild). But people rarely are willing to spend all of their time holding your hand through 20 missions straight.

8. There are a lot of mechanics that one could discuss. Maybe the most important is: Spiral Knights is fundamentally about dodging. You are supposed to stay mobile, avoid getting hit, and continually inflict damage or status on your enemies. Your shield is a marvelously subtle tool with tricks such as shield-canceling and shield-bumping. But don't rely on your shield for defending your body.

9. To get crowns, see my FAQ linked above. It tells you general principles and includes links to data sets.

1) It's reasonably short so long as you decide to play most missions on Normal mode; you should only have problems reaching 4* and 5* certifications, and even then they should only be minor stops so long as you take Points 2), 6) & 9) into consideration.

2) I'd suggest you do some research on what type of equipment you wanna use (especially swords/handguns/bombs) so that you make the best out of what the game hands you out as "mission rewards", but to put it simply, it's much easier to optimize your gear as a single-type specialist (AKA swordsman or gunner or bomber) than as a hybrid.

3) Some guilds may expect you to reach Vanguard rank or have some T3 equipment before you can join them, but besides that, "time" and "experience" aren't really valued in them; just pray you find the right guild for you in your first try and it sticks around long enough with hardly any administrative changes, lest you end up like me and make your own one-man guild simply out of spite for all the terrible ones you had sour experiences with.

4) The playerbase is, in a nutshell, more stable on the veteran side than the newcomer one; you'll be lucky if you find a free party on an Arcade run or a popular mission, and you'll be even luckier if you find enough people to do more than 1 game of Lockdown at once -- much less Blast Network -- but you'll always find an interesting mix of active guilds roaming around the clockworks together, there always tend to be at least 1 T3 LD game going while you're waiting alone, and there are always will be enough people to fill at least 1 server room of Haven('s Auction House) close to full. It's interestingly alienating, in a way.

5) Most people nowadays have moved on towards the English server rooms, whether they actually speak English or not; just expect some foreign language to pop up on global chat every now and again, even if the majority speaks English.

6) Unless you wanna heat your Proto Gear, leveling up early-game gear is a moot point as you hardly need any non-T3 fire crystals to level up and upgrade your equipment; I'd suggest you get mentally ready for whenever you need to grind for Shining and especially Radiant Fire Crystals (4* and 5* respectively), and make sure to use your Pokémon intuition to know when forging odds are consistent enough or a shot in the dark -- Pro Tip: any forging odds below 80% are essentially a luck of the draw and not consistent enough to use your well-earned crystals on.

7) People are usually nice enough to hand you out some non-5* materials or other insignificant stuff if you are in need, but besides that, they either don't care much about new players or they outright get disgusted at them if they beg for money/energy/rarities/accessories -- Pro Tip: begging for any of those things is tantamount to receive a slap on the face, so don't do it.

8) Shield-bashing is a good way to start battles by stunning enemies while they just spawned and can't do much to you; Dashing is amazing in giving you some breathing room as it gives you a few invincibility frames and temporarily removes aggro on enemies, all the while letting you escape dicey moments where your shield keeps taking multiple hits at once; Shield-bumping, which is what happens when you raise your Shield near an enemy or block its melee attack, is also excellent at giving you room to breathe and is a tad more spammable than Dashing; Shield-cancelling, which is what happens when you raise your Shield in the midst of using a weapon, allows you to remove ending lag on your attacks and replace them with safe frames while behind your Shield, and is a staple in swordfighting and gunslinging.

9) The most efficient ways to get Crowns are from Major Boss Missions, with "The Sovereign Slime" (Royal Jelly Palace) and especially "The King of Ashes" (FireStorm Citadel) standing out as the most popular grinding spots, but them being pretty long can be quite tiring. I've been hearing good things about the "Plan of Attack" mission lately though, like how it's a repeatable two-floor mission that has pretty decent payout for its kind, so you could always try that one while you're cooling down from all the FSC grinding. Either way, you're gonna have to be a man and play these missions on Elite difficulty in order to make the most of your grinding.