Disclaimer: I am not a lawyer and this is not legal advice.
A port to another language is a translation, and, a translation is considered a derivative work, which means the original copyright (whatever it is) applies (e.g. if you have limited rights to the original so do you to the translation).
As GPL2 is copyleft license, you must reproduce the full CompanyX license text somewhere within your project. Typically this would be placed at the top level of the project, e.g. perhaps alongside a README file. (Reproduction of the CompanyX license is necessary in both source distributions and binary distributions.)
You / your company will also be copyright holders for the translation you have made. Thus, you / your company will also provide a license.
In effect copyrights for the translation will be held by two (or more) parties. (Here is where it is necessary that the various licenses are compatible, e.g. don't contradict each other.)
(If there are different licensing arrangements for different parts of the project (say you merged two license compatible (e.g. GPL) projects), this should be called out as such, and if there aren't then I would probably explicitly call it out that these licenses cover all files in the project.)
Without being a lawyer or giving legal advice: I personally believe that the above alone covers your legal obligations to CompanyX and your legal needs for OurCompany — so, what you put in the individual files (as a copyright line and licensing reminder) is more or less up to you.
As the proximal authors of the translation, I would suggest to put within each file of your work, a statement of copyright to your company (and date of the translated work), along with a statement referencing the complete & detailed licensing situation that can be found at the top level of the project — it might be polite to call out that this is a translated work and thus has multiple copyright holders.
As far as I know, I don't think there is any requirement to make an explicit copyright statement for ComanyX within each file that is translated. (I'm not even sure if you have the moral authority to do so, as this might imply to others that the original authors were more involved in the translation.)
For original CompanyX files reproduced as is, I would just leave them alone, having already reproduced their license at the top-level of your project (and made clear which files/directories (e.g. all) are under which licenses).