I have the same issue.

Some investigations:
  1. I decided to find out the particular command that fails and added more debug print:

    diff --git a/bootstrap b/bootstrap
    index 7523f65b4..44c21db23 100755
    --- a/bootstrap
    +++ b/bootstrap
    @@ -749,6 +749,7 @@ download_po_files() {
       domain=$2
       echo "$me: getting translations into $subdir for $domain..."
       cmd=$(printf "$po_download_command_format" "$subdir" "$domain")
    +  echo "$me: going to exec \"$cmd\"..."
       eval "$cmd"
    }

  2. Tried to run:

    $ ./bootstrap
    ./bootstrap: Bootstrapping from checked-out coreutils sources...
    ./bootstrap: consider installing git-merge-changelog from gnulib
    ./bootstrap: getting gnulib files...
    ./bootstrap: getting translations into po/.reference for coreutils...
    ./bootstrap: going to exec "wget --mirror --level=1 -nd -nv -A.po -P 'po/.reference' https://translationproject.org/latest/coreutils/"...
    ERROR: The certificate of 'translationproject.org' is not trusted.
    ERROR: The certificate of 'translationproject.org' doesn't have a known issuer.

  3. Tried to run the command directly, but without `-nv` flag:

    $ wget --mirror --level=1 -nd -v -A.po -P 'po/.reference'  https://translationproject.org/latest/coreutils/
    --2021-02-13 14:23:35--  https://translationproject.org/latest/coreutils/
    Loaded CA certificate '/etc/ssl/certs/ca-certificates.crt'
    Resolving translationproject.org (translationproject.org)... 80.69.83.146, 2a01:7c8:c037:6::20
    Connecting to translationproject.org (translationproject.org)|80.69.83.146|:443... connected.
    ERROR: The certificate of ‘translationproject.org’ is not trusted.
    ERROR: The certificate of ‘translationproject.org’ doesn't have a known issuer.

  4. Tried the same with curl:

    $ curl -v https://translationproject.org/latest/coreutils/ -o /dev/null
      % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                     Dload  Upload   Total   Spent    Left  Speed
      0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0*   Trying 80.69.83.146:443...
    * Connected to translationproject.org (80.69.83.146) port 443 (#0)
    * ALPN, offering h2
    * ALPN, offering http/1.1
    * successfully set certificate verify locations:
    *  CAfile: /etc/ssl/certs/ca-certificates.crt
    *  CApath: none
    } [5 bytes data]
    * TLSv1.3 (OUT), TLS handshake, Client hello (1):
    } [512 bytes data]
    * TLSv1.3 (IN), TLS handshake, Server hello (2):
    { [93 bytes data]
    * TLSv1.2 (IN), TLS handshake, Certificate (11):
    { [6723 bytes data]
    * TLSv1.2 (IN), TLS handshake, Server key exchange (12):
    { [589 bytes data]
    * TLSv1.2 (IN), TLS handshake, Server finished (14):
    { [4 bytes data]
    * TLSv1.2 (OUT), TLS handshake, Client key exchange (16):
    } [70 bytes data]
    * TLSv1.2 (OUT), TLS change cipher, Change cipher spec (1):
    } [1 bytes data]
    * TLSv1.2 (OUT), TLS handshake, Finished (20):
    } [16 bytes data]
    * TLSv1.2 (IN), TLS handshake, Finished (20):
    { [16 bytes data]
    * SSL connection using TLSv1.2 / ECDHE-RSA-AES128-GCM-SHA256
    * ALPN, server did not agree to a protocol
    * Server certificate:
    *  subject: CN=stats.vrijschrift.org
    *  start date: Dec 31 10:34:41 2020 GMT
    *  expire date: Mar 31 10:34:41 2021 GMT
    *  subjectAltName: host "translationproject.org" matched cert's
    "translationproject.org"
    *  issuer: C=US; O=Let's Encrypt; CN=R3
    *  SSL certificate verify ok.
    } [5 bytes data]
    > GET /latest/coreutils/ HTTP/1.1
    > Host: translationproject.org
    > User-Agent: curl/7.75.0
    > Accept: */*
    >
    { [5 bytes data]
    * Mark bundle as not supporting multiuse
    < HTTP/1.1 200 OK
    < Date: Sat, 13 Feb 2021 12:26:00 GMT
    < Server: Apache/2.4.10 (Debian)
    < Vary: Accept-Encoding
    < Transfer-Encoding: chunked
    < Content-Type: text/html;charset=UTF-8
    <
    { [5 bytes data]
    100  8881    0  8881    0     0  16980      0 --:--:-- --:--:-- --:--:-- 16980
    * Connection #0 to host translationproject.org left intact

  5. Trying to export and verify the cert with certtools:

    $ certtool --verbose --verify-profile=high --verify --infile=/tmp/stats.vrijschrift.org
    Loaded system trust (139 CAs available)
            Subject: CN=R3,O=Let's Encrypt,C=US
            Issuer: CN=DST Root CA X3,O=Digital Signature Trust Co.
            Signature algorithm: RSA-SHA256
            Output: Not verified. The certificate is NOT trusted. The certificate issuer is unknown.

            Subject: CN=R3,O=Let's Encrypt,C=US
            Issuer: CN=DST Root CA X3,O=Digital Signature Trust Co.
            Signature algorithm: RSA-SHA256
            Output: Not verified. The certificate is NOT trusted. The certificate issuer is unknown.

            Subject: CN=R3,O=Let's Encrypt,C=US
            Issuer: CN=DST Root CA X3,O=Digital Signature Trust Co.
            Checked against: CN=DST Root CA X3,O=Digital Signature Trust Co.
            Signature algorithm: RSA-SHA256
            Output: Verified. The certificate is trusted.

            Subject: CN=stats.vrijschrift.org
            Issuer: CN=R3,O=Let's Encrypt,C=US
            Checked against: CN=R3,O=Let's Encrypt,C=US
            Signature algorithm: RSA-SHA256
            Output: Verified. The certificate is trusted.

    Chain verification output: Verified. The certificate is trusted.

    Maybe that "Output: Not verified. The certificate is NOT trusted. The certificate issuer is unknown." Is the issue?

Thanks!
Best regards,
Grigorii