First, check in romcenter if your transation is not yet available.

Romcenter strings are stored in a resource file located in romcenter\resx folder. You can edit existing files or create a new one by copying an romcenter.resx and adding your language code.

With a text editor With a resource editor: There are several free resx editors. I recommend using the standalone version of (https://github.com/tom-englert/ResXResourceManager/releases). It can do an automatic pre-translation using google translate. Other tool: Zeta Resource Editor

Once the translation is done, you must compile the resx file to a satellite assembly for romcenter to use it.

    .resx --(resgen.exe)-> .resources --(al.exe)-> .dll

Commands to generate a satellite assembly from a string resource file:

Replace the XX by your language code (see here:http://www.csharp-examples.net/culture-names/)

resgen.exe /useSourcePath /compile romcenter.XX.resx,RomCenter.Localization.Resx.romcenter.XX.resources

al.exe /culture:XX /out:RomCenter.Localization.resources.dll /platform:AnyCPU /template:..\RomCenter.Localization.dll /embed:RomCenter.Localization.Resx.romcenter.XX.resources

This will generate the assembly RomCenter.Localization.dll

Copy this file into romcenter\XX\
Run romcenter
Go into File/options and change language.      

Help and support: http://forum.romcenter.com