This is the problem (update button for localized WP is missing):

And this should be correct content for a localized site:

It is not easy to reproduce, but I have many websites (ca. 200) on the same hoster without such behaviour.
If I disable every plugin (with Site Health Troubleshooting Mode) the error is gone and I see the button and it is not consistent when the button disappears again. It seems to be a conflict with the Theme Update plugin (because it is the only plugin which is interfering with the update system), but sometimes it works again if this pugin is disabled …
Here is a report of the server and site config:
### wp-core ###
version: 5.2.5
site_language: de_DE
user_language: de_DE
permalink: /%year%/%monthnum%/%day%/%postname%/
https_status: true
user_registration: 0
default_comment_status: closed
multisite: false
user_count: 1
dotorg_communication: true
### wp-active-theme ###
name: Fiji 2 (fiji2)
version: 1.2.2
author: Flothemes
author_website: https://flothemes.com
parent_theme: none
theme_features: wc-product-gallery-zoom, wc-product-gallery-lightbox, wc-product-gallery-slider, post-formats, widgets, post-thumbnails, menus, html5, title-tag, automatic-feed-links, custom-background, woocommerce
theme_path: /www/htdocs/[...]/wp/wp-content/themes/fiji2
### wp-themes-inactive (4) ###
Twenty Fifteen: version: 2.5, author: WordPress-Team
Twenty Nineteen: version: 1.4, author: WordPress-Team
Twenty Seventeen: version: 2.2, author: WordPress-Team
Twenty Sixteen: version: 2.0, author: WordPress-Team
### wp-mu-plugins (2) ###
Health Check Troubleshooting Mode: author: (undefined), version: 1.7.0
InfiniteWP - Client Loader: author: Revmakx, version: (undefined)
### wp-plugins-active (12) ###
BackWPup: version: 3.7.0, author: Inpsyde GmbH
Classic Editor: version: 1.5, author: WordPress Contributors
Flex Block: version: 2.0.2, author: Flothemes
Flo Footer Credits: version: 0.1, author: Alexandru Guzun
Flo Forms: version: 1.0.16, author: Flothemes
FloSupport: version: 0.4, author: Flothemes.com
Flothemes Theme Updater: version: 2.1, author: Flosites | George M.
Health Check & Troubleshooting: version: 1.4.2, author: The www.ads-software.com community
InfiniteWP - Client: version: 1.9.4.4, author: Revmakx
Simple Custom Post Order: version: 2.4.7, author: Colorlib
Transients Manager: version: 1.8, author: Pippin Williamson
WPFront Scroll Top: version: 2.0.2, author: Syam Mohan
### wp-plugins-inactive (2) ###
Statify: version: 1.6.3, author: pluginkollektiv
Statify – Extended Evaluation: version: 2.6.0, author: Patrick Robrecht
### wp-media ###
image_editor: WP_Image_Editor_Imagick
imagick_module_version: 1673
imagemagick_version: ImageMagick 6.8.9-9 Q16 x86_64 2019-11-12 https://www.imagemagick.org
imagick_limits:
imagick::RESOURCETYPE_AREA: 31 GB
imagick::RESOURCETYPE_DISK: -1
imagick::RESOURCETYPE_FILE: 48750
imagick::RESOURCETYPE_MAP: 31 GB
imagick::RESOURCETYPE_MEMORY: 16 GB
imagick::RESOURCETYPE_THREAD: 8
gd_version: bundled (2.1.0 compatible)
ghostscript_version: 9.26
### wp-server ###
server_architecture: Linux 4.4.0-170-generic x86_64
httpd_software: Apache
php_version: 7.2.24-nmm1 64bit
php_sapi: fpm-fcgi
max_input_variables: 3000
time_limit: 30
memory_limit: 512M
max_input_time: 60
upload_max_size: 200M
php_post_max_size: 200M
curl_version: 7.47.0 OpenSSL/1.0.2g
suhosin: false
imagick_availability: true
server-headers:
date: Wed, 18 Dec 2019 15:10:46 GMT
server: Apache
expires: Wed, 11 Jan 1984 05:00:00 GMT
cache-control: no-cache, must-revalidate, max-age=0
link: Array
upgrade: h2,h2c
vary: Accept-Encoding
content-encoding: gzip
content-type: text/html; charset=UTF-8
htaccess_extra_rules: true
### wp-database ###
extension: mysqli
server_version: 5.7.28-nmm1-log
client_version: mysqlnd 5.0.12-dev - 20150407 - $Id: 3591daad22de08524295e1bd073aceeff11e6579 $
### wp-constants ###
WP_HOME: undefined
WP_SITEURL: undefined
WP_CONTENT_DIR: /www/htdocs/[...]/wp/wp-content
WP_PLUGIN_DIR: /www/htdocs/[...]/wp/wp-content/plugins
WP_MAX_MEMORY_LIMIT: 256M
WP_DEBUG: false
WP_DEBUG_DISPLAY: true
WP_DEBUG_LOG: false
SCRIPT_DEBUG: false
WP_CACHE: false
CONCATENATE_SCRIPTS: undefined
COMPRESS_SCRIPTS: undefined
COMPRESS_CSS: undefined
WP_LOCAL_DEV: undefined
### wp-filesystem ###
wordpress: writable
wp-content: writable
uploads: writable
plugins: writable
themes: writable
mu-plugins: writable
Any help is much appreciated!
All the best,
Torsten
PS: The other error is re-appearing after each plugin deacivation and re-activation.