Some strings are not translated because they are not currently available on Crowdin. I'm talking about:
Presets in Superflat Customization > Select a Preset that are in English and aren't translatable on Crowdin.fixed in 1.11The text in debug menu (F3) is also not translated.Mod edit: It's a debug screen, only for testing purposes, not for normal gameplay.The biomes in the biome slider (Customized world) also doesn't have translation.Invalid according to @unknown.The Elder Guardian as well – Unable to find this one @unknown.
Default empty world name is "World"Invalid according to @unknown.Commands in spectator mode ("Press a key to select a command, and again to use it.", "Teleport to player", "Teleport to team member", "Next Page", "Close menu")fixed in 1.11-Options -> Graphics settings
> "Entity Shadows"Fixed in 1.9New 1.8.2 / 1.8.3 Statistics (MC-77880)Fixed in 1.9Potion item does not translate since the removal of data valueFixed in 1.9The new error message added in 16w02a: "Invalid damage property. Please pick in [Valid Blockstate Numbers]"Message got removedThe new debuging messages, for example "[Debug]: Reloading all chunks"fixed in 1.11A number of subtitles: Are at cowdin, but not translated in game yetFixed in 1.9.2Video settings: chunks and FPSfixed in 1.11"Color" on dyed armor in advanced tooltipsfixed in 1.11"NBT x tag(s)" in advanced tooltipsfixed in 1.11"Unable to load world" screen is not translated at all (outside of the cancel button)fixed in 1.11
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As of August 24 2014 the flatland preset stings are available at crowdin: createWorld.customize.custom.preset.*
The debug screen is a debug screen and will never get translated.
Remains the shader button - but I doubt that this will get a translation.
Where did you get those info?

Yes but I talked about the superflat presets that are still not available on Crowdin.

Oh crap. Mistook the customized presets for the flatland presets.

The biomes in the biome slider (Customized world) also don't have translation.

Two more exploits!
First: When you create a world and remove everything from the world name's text box, the name will always default to "World", even in non-English languages.
Second: This one is more serious. The commands in spectator mode can't be translated ("Press a key to select a command, and again to use it.", "Teleport to player", "Teleport to team member", "Next Page", "Close menu"). I swear they added this as a test feature, and said: "Oh, I don't even need to make this translatable since I'll change it a lot until it's done", and then they forgot to finish it.
Also, confirmed for every new version until 1.8.2-pre7.
@Bentroen
Texts from upcoming versions will not be translated until the upcoming versions are released.
These texts are from 1.8, which was already released.

Exactly! And it looks like "Entity Shadows" will be a forgotten one also (as well as the new statistics, which are already featured in an official release and didn't get anything on Crowdin). And...
Third: The text shown in the Minecraft default resource pack can't be translated.
Fourth: The "NONE" tag in the Controls screen can't also.
Wow... And confirmed for 1.8.2 and 1.8.3.

Third: The text shown in the Minecraft default resource pack can't be translated.
This is not a bug, but working by design. It cannot be translated since not in the language file and cannot be put there (like the description of every other resource pack)
Fourth: The "NONE" tag in the Controls screen can't also.
Most probably hardcoded in LWJGL like "LCONTROL" and "SPACE"

Oh, yeah, most likely. But I don't think there isn't any possible ways of converting the tags in LWJGL to strings so that they appear correctly in-game. That's kinda the same thing that should be done with the biomes in the biome slider, since the names that appear there are the technical names of the biomes, if I'm correct.
I also don't think that the resource pack text can't be fixed. It's impossible that a replacement text can't be put in the place of the current string from the MCMETA file. Right?

Fifth: The "and" in the /say command using selectors can't be translated.
Sixth: The "and" in the output of /gamerule, that lists all the gamerules.
Sorry for the amount of comments I added in the last days. I'll try to edit some of them when I need to add something.

Confirmed for 1.8.8, 15w36d and 15w37a.
super secret settings button is no longer an issue due to it's removal
Potion item does not translate since the removal of data value
Ofcourse confirmed for 16w02a
Also, the new error message added in 16w02a,
Invalid damage property. Please pick in [Valid Blockstate Numbers]
,
and also the new debuging messages for example
[Debug]: Reloading all chunks
is untranslatable.
So, the new error message was actually removed, so no translations are needed.
However, new things are still untranslated (which will be probably fixed in 1.10).
These are the following untranslated subtitles that I've found.
subtitles.ambient.cave=Eerie noise
subtitles.block.chest.locked=Chest locked
subtitles.block.tripwire.click=Tripwire clicks
subtitles.block.tripwire.detach=Tripwire detaches
subtitles.entity.bat.takeoff=Bat takes off
subtitles.entity.blaze.burn=Blaze crackles
subtitles.entity.bobber.throw=Bobber thrown
subtitles.entity.donkey.angry=Donkey neighs
subtitles.entity.donkey.chest=Donkey Chest equips
subtitles.entity.endereye.launch=Endereye shoots
subtitles.entity.generic.extinguish_fire=Fire extinguishes
subtitles.entity.horse.angry=Horse neighs
subtitles.entity.horse.armor=Horse armor equips
subtitles.entity.horse.breathe=Horse breathes
subtitles.entity.horse.saddle=Saddle equips
subtitles.entity.pig.saddle=Saddle equips
subtitles.entity.player.levelup=Player dings
subtitles.entity.villager.no=Villager disagrees
subtitles.entity.villager.trading=Villager trades
subtitles.entity.villager.yes=Villager agrees
subtitles.entity.wolf.shake=Wolf shakes
subtitles.entity.zombie.infect=Zombie infects
And also, confirmed for 1.9
Actually, the subtitles you named can already be translated on Crowdin. In the next language update, the translations will be in the game.
Also, the Potion items are actually translatable and translated in 1.9.
Why is it marked as fixed? actually, there are still some text that has not been uploaded to Crowdin
I just realized that "chunks" and "fps" in the video settings can't be translated too. (1.9.2)
Also i saw that "tag(s)" under Adv. Tooltips that says "NBT: x tag(s)" is not translated too. Not sure if it should tho.
The strings which have been added in 1.9.4
commands.stopsound.usage=/stopsound <player> [source] [sound]
commands.stopsound.unknownSoundSource=Source %s is unknown
commands.stopsound.success.individualSound=Stopped sound '%s' with source '%s' for %s
commands.stopsound.success.soundSource=Stopped source '%s' for %s
commands.stopsound.success.all=Stopped all sounds for %s
can't be translated on Crowdin either. Also, confirmed for 1.9.4.
Honestly, now I think this is more a problem of the Crowdin page rather than Minecraft itself. Self-explanatory I guess.
Well, you have a point there, but this is not entirely a Crowdin issue.
Some of the strings which can't be translated can't be translated because they're hardcoded (such as "chunks" and "fps" in the video settings). And those which are already in the translation file for English have to be added to Crowdin manually by Jeb or another Mojang member. So, this bug report is about both the hardcoded translations as well as Mojang (sometimes) forgetting to update the translations over there at Crowdin.
Still in 16w20a
Still in 16w21a and 16w21b
Still in 1.10-pre1

/stopsound-related messages are translatable now, others remain.
Fixed are:
The new debuging messages, for example "[Debug]: Reloading all chunks"
Presets in Superflat Customization > Select a Preset that are in English and aren't translatable on Crowdin.
"Unable to load world" screen is not translated at all (outside of the cancel button)
Commands in spectator mode ("Press a key to select a command, and again to use it.", "Teleport to player", "Teleport to team member", "Next Page", "Close menu")
Unchanged are:
"Color" on dyed armor in advanced tooltips
"tags" in advanced tooltips
Invalid are:
Default empty world name is "World"
The text in debug menu (F3) is also not translated.
The biomes in the biome slider (Customized world) also doesn't have translation.
Couldn't find these?
The Elder Guardian as well
Video settings: chunks and FPS

chunk
and fps
are in the video settings on the top right.
Also fixed:
Video settings: chunks and FPS
"Color" on dyed armor in advanced tooltips
"NBT x tag(s)" in advanced tooltips
And a missed one:
"Durability: x / x" on advanced tooltips
"Scaling at 1:x" and "Level x/x" from map advanced tooltips are still missing.
I created a new ticket for those: MC-109871.
Confirmed.