Thank you very much for your help and advise.
As per attached investigation and solution.
This is standard SAP in customizing.
Equipment category M does not have the multilingual text checked, which means that which ever language login, the creation of the equipment category M will only have one language, which is the login language as default.
And you are right that equipment category M can only have one language.
Therefore, if the person who have equipment category M created in language CS (Czech), the default one language created in the equipment will be CS. If login to EN (english) login, you will not be able to see the text in english as this is not created in the equipment category M.
However, if you would like to see in EN login the equipment category M text with CS language created, please also see the attached solution on how to rectify this. If there is a lot of equipment category M that does not have EN text to be seen in EN login, we can also use LSMW to update the equipment.
Please advise whether you would like to add in more language to the equipments.