You're talking about a new wallet right?

That is the actual use-case of the "seed word extension" so, yes.
But I'd make a separate offline backup of the passphrase (the extra word) just in case because it's not really reliable to keep it in one's memory alone.

It's because it wont work in the first place.
In BIP39, the "mnemonic sentence" itself will be used to generate the seed (binary seed), not the 'entropy' where the words are based.
So if you translate them just based from the position in the wordlist, it will generate a valid seed phrase but an entirely different wallet.

What you want to do is to paste your BIP39 seed phrase to an offline instance of iancoleman's BIP39 tool (iancoleman.io/bip39/, read "Offline usage").
Then select BIP44 tab below, and copy "Account Extended Private Key" that starts with "xprv".
Use that to restore your wallet to your Electrum by selecting: "Standard wallet->Use a master key" and it should generate the same set of addresses as long your Copay wallet is using the default derivation path for BIP44.

(see when you change language in 'BIP39 tool', it uses the same entropy to translate the seed phrase in other language but the "BIP39 Seed" below it changes)

Just adding this from the post above posted by hosseinimr93

If you can't find the right derivation path of your wallet you can use a tool like https://www.blockpath.com/r/Blockpath/comments/2r/blockpath_app_xpub_analyzer/

Since you already have the master public key you can paste it on that tool to find used addresses that includes all exact derivation path.

In electrum, go to "Addresses" tab and check your addresses there. Are they same as the addresses Copay give you?
If no, the problem may be with the derivation path. There's a possibility that copay has generated the addresses on a different derivation path.

Note: If "Addresses" tab is not available in electrum, click on "View" at top of the window and select "Show addresses".

Your seed is a BIP39 one, isn't it?

If you don't want to go with that way, which is not recommended unless you want to save some sats from the transaction fee, you should do the following;

1) Open bip-0039.
2) Open two tabs, one will be the English wordlist and the other your language's wordlist.
3) Find the line of your first word from your language's mnemonic. Then go on that line from the english wordlist. That will be your english word. Redo it for every word.

Example:

Ubuntu has the advantage of being quite popular so there are literally hundreds of guides around on how to install, configure and use it...

Are you considering using Electrum as your airgapped wallet? If so, the Electrum docs have an ok guide on how to set up the wallet side of things after you get your OS etc sorted. https://electrum.readthedocs.io/en/latest/coldstorage.html

Basically, you just create a "standard" wallet on the airgapped machine, then a "watching-only" wallet using the "xpub" on your online machine... It's not terribly difficult.

The real issue is making sure you properly airgap the machine (ensuring no networking/wifi etc after the OS is setup and BEFORE you create the wallets)

Your seed is a BIP39 one, isn't it?

If you don't want to go with that way, which is not recommended unless you want to save some sats from the transaction fee, you should do the following;

1) Open bip-0039.
2) Open two tabs, one will be the English wordlist and the other your language's wordlist.
3) Find the line of your first word from your language's mnemonic. Then go on that line from the english wordlist. That will be your english word. Redo it for every word.

Example:

Do you have any experience with Linux? If not, then Linux Mint is probably the most user friendly, especially if you are coming from a life of using Windows. Elementary OS is probably the best if you are more familiar with MacOS. If you think you can handle something a little more, then Ubuntu is probably the most well known and widely used distro.

If you want to go for maximum security and privacy for your airgapped wallet, though, then I would probably opt for Tails. There is a bit of learning curve here though, as Tails is designed to leave no traces behind and will not save anything between sessions unless you specifically set up persistent storage. Tails will automatically encrypt your persistent storage. You can read a guide here: https://tails.boum.org/doc/first_steps/persistence/index.en.html

If you don't use Tails, then you will need to look in to encrypting your wallet file some other way, as otherwise anyone who can gain physical access to your airgapped computer could steal all your coins.

I'm sure other users will completely disagree with me and can provide other recommendations though. Such is the nature of the myriad of Linux distros.

Why not download and verify the desktop version, create a legacy wallet, send the funds there, then immediately send them out to a segwit mobile wallet, if that's where you want your coins to ultimately end up. Wait until the fees are lower to do this though if you are not in a hurry, to save yourself as much as possible on the two transactions.

This all depends. What are you planning to do with the seed phrase once you have created it? Are you making a paper wallet? Do you want to import the seed phrase to a wallet you can access?
You are thinking along the right lines, but you have got some things incorrect. The OS you use should not be Windows, but rather an open source Linux distro. You shouldn't download anything to the computer after you have formatted it - it shouldn't even have an internet connection. You should remove any WiFi hardware and unplug any ethernet cables or similar before you start the process to ensure it is permanently disconnected from the internet. Any software, such as Electrum, should be downloaded and verified on a different device and then transferred to your airgapped device using a USB drive.

Why not download and verify the desktop version, create a legacy wallet, send the funds there, then immediately send them out to a segwit mobile wallet, if that's where you want your coins to ultimately end up. Wait until the fees are lower to do this though if you are not in a hurry, to save yourself as much as possible on the two transactions.

This all depends. What are you planning to do with the seed phrase once you have created it? Are you making a paper wallet? Do you want to import the seed phrase to a wallet you can access?
You are thinking along the right lines, but you have got some things incorrect. The OS you use should not be Windows, but rather an open source Linux distro. You shouldn't download anything to the computer after you have formatted it - it shouldn't even have an internet connection. You should remove any WiFi hardware and unplug any ethernet cables or similar before you start the process to ensure it is permanently disconnected from the internet. Any software, such as Electrum, should be downloaded and verified on a different device and then transferred to your airgapped device using a USB drive.

You can't.  

Yes, there is. If you're on PC click on View —> Show Console and then type on it:

Take the seed that it'll return you, click on File —> New/Restore —> Next —> Standard Wallet —> I already have a seed —> enter it —> Next and there you have it. A hierarchical deterministic wallet with only legacy addresses.

Creating a seed on the PC version of Electrum and restoring it to your mobile wallet as HCP and BlackHatCoiner have suggested is probably the easiest solution to get a legacy wallet on your phone. There is no way to directly create a legacy wallet on the latest versions of the Android app. What you could do, however, if you have no access to a desktop computer, is uninstall your Android app (make sure you have the seed phrase to any wallets you have created first!), and then install a version from before 4.1.0, which is when the option to create legacy wallets was removed. Once you have created your legacy wallet, you can then update again to the latest version and your legacy wallet will still be accessible.

You'll need to download the appropriate .apk file to your phone from here - https://download.electrum.org/4.0.9/. You'll want the arm64 file for a 64 bit OS and armeabi one for a 32 bit OS. You'll then need to go in to your phone settings and allow "installing from unknown sources", or something similar, before executing the file.

It actually does; what it does not accept is legacy invoices. You can normally send BTC to a legacy address, what you can't do is generate one. May I ask why you want to generate one? You'll have to pay a greater fee for each transaction you make. The SegWit war didn't happen arbitrarily. 

But, to answer your question I think the Bitcoin Wallet, from play store, still allows you to generate those.