Ok, if no one with good English wants to reply to this post then I'll try.
When you compress audio with side-chain compressor and you have eq that is for example boosting 6KHz in the side-chain path then this compressor is more sessitive to that part of the frequency spectrum. If you cut the frequencies that are above and below 6KHz with that eq the compressor will attenuate only when frequencies in the 6KHz region are present in the side-chain path. If that happens the compressor will attenuate the whole spectrum of frequencies. If you use multiband compressor and set the tresholds so that only one narrow band (say 6KHz) is working then only this part of the spectrum is attenuated. That is the difference.

If it is similar, can you use a multiband compressor as a de-esser?

Most de-essers are specially "tuned" compressors (wideband and multiband). You can use both multiband and wideband compressors as de-essers but widebands need to have eq in sidechain path for that purpose.
There is no one right way to de-ess. Sometimes its better to do it with automation in your DAW. English is relatively easy :wink: to de-ess. It's quite hard to polish Polish vocals. :(

So would that mean that adding an EQ before a compressor is the same as having the same EQ in a side-chain?

So would that mean that adding an EQ before a compressor is the same as having the same EQ in a side-chain?

No, absolutely not. Eq boost before the comp means more of that frequency is in your sound after comp but that desn't have to mean that comp would react more to just that frequency. For example boost 6KHz in the eq before comp on the kick track and you'll have more 6KHz on the output. The compressor will react mostly to the low frequencies that dominate the spectrum in kick drum. Do the same boost in the side-chain and you wont get more of 6KHz in the output. Besides, boosting anything in the side-chain usualy means that it's lower in the comps output. Please correct me if I'm wrong.