(german LibreOffice version 7.1)
If I have a number like:
123,45
any double click selection on decimal parts “123” or “45” Writer selects the WHOLE number/text … which is not desired.
I usually want to select only the one part (where I clicked), either “123” or “45”.
How to achieve hat?
Btw, if the number format uses the point:
123.45
the selection is OK 
(but I need “,” as separator)
Selecting as Decimal separator key “.” or “,”, Writer and Calc selects 123.45 as two words, and 123,45 as one.
Ctrl+Left arrow and Ctrl+Right arrow shows a similar behavior.
Definition of a “word” depends on the locale.
With locale “deutsch (Deutschland)”, numbers are written with a comma and a sequence like <digits>,<digits> is considered a word and selected as a whole by double-click. On the contrary, a number with a dot doesn’t comply with German grammar and will be decomposed into three parts integer, dot, fractional part. Double-click will only select a single part.
If this does not work, check various configurations:
Tools>Options, Language Settings>Languages where:
If you frequently write in a language different from the default for the OS, you can change LO configuration (but it will be changed for all future documents) or change the language in paragraph style Default Paragraph Style, Font tab (this change will cascade to all other styles).
You can also prepare a custom template for each of your languages and create new documents from this template.
Thanks for the suggestions.
My OS: Windows 10, US english, regional format: german/germany.
I tried everything … Switzerland, Germany, USA, dialog options ON and OFF … no changes double click in selection/behavior with “,”.
A template would help in future documents, but I have many old documents.
I doubt this specific selection depends on locale; I tried in en-GB (. dot decimal separator and , comma group separator) also with paragraph locale set to en-GB and still 123,45 is selected with a double click but not 123.45.
I made my test under various locales and, indeed under Linux, it makes a difference. So, there may be some dependency on the OS, but I don’t see why as I guess double-click selection is local to LO and not an OS-service.
There may also be a dependency on the desktop manager. Mine is KDE Plasma.
Where/how did you change locales? I tried on Linux under GNOME using a LibreOffice English-US UI and the default English-UK locale following the en-GB system locale and English-UK default document language and it didn’t make a difference to what the OP experienced.
Locale was changed with “system settings”. As mentioned, I’m not under Gnome but under KDE Plasma. This may make a difference. LO has Same as locale setting for Decimal separator key in Language Settings>Languages.
Changing locale in OS immediately takes effect in Writer without the need to reboot or exit-relaunch LO (which surprised me). There may be some DBus notification behind the scene.
OS is Fedora 35 (on standby to upgrade to Fedora 36 because I noticed some problems on my laptop).
Confirming that changing locales in LibreOffice on Windows (ru-RU using comma, and en-UK/en-US using dot) does not change the behaviour - I repro OP’s experience.
Also I believe that if something in LibreOffice would affect that, that needed to be text language, which is what defines word boundaries; I also don’t see setting text language to the mentioned ru-RU and en-UK making any difference.