Thar Archie comics including the character of Jughead were fist published in 1942. They are still protected by copyright in the US, and almost certifiably in Finland as well, depending on who is considered the original author or authors, as Finland uses a Life+70 term, even for corporate copyrights, when an author can be identified.
The name "Jughead" is not protected by copyright, either under US law, or under the law of Finland. Names and short phrases are not protected by copyright in any country signatory to the Bern convention, which includes both the US and Finland.
The typical image of the Jughead character will be protected by copyright. Neither a copy of an image from the comics, nor a new image derived from that source may be used without permission from the copyright holder, which is probably Archie Comics, inc.
Use of a name such as "Jughead's" or "Jughead's Diner" without an image or logo, or with an image in no way derived from any image published as part of the comics, would not infringe any copyrights.
Trade Mark Issues
Protection of the name "Jughead" as a trademark in the US (which is likely but I have not confirmed it) is irrelevant. Only protection under the law of Finland will matter.
Protection can be obtained by registration with the Finnish Patent and Registration Office, or by an EU-wide registration.
In addition, protection can be obtained if a mark is "established in Finland." This happens when a mark "has become generally known in the relevant commercial or consumer sectors in Finland as a specific symbol of the proprietor’s goods." (See the WTR page below for more on this.) The WTR page says that for such establishment to occur:
the relevant group is required to associate the unregistered mark with the entity from which the goods or services originate. The target group needs to have a sufficient degree of awareness of the mark. This is assessed individually for each case and the required level of use depends heavily on the target group for the relevant goods or services. However, as a general principle the use must be nationwide and extensive.
So unless "Jughead" is registered in Finland, or in the EU-wide system, or is sufficiently well-known in Finland to be "established" there, there is no trademark protection for that mark in Finland, and using it is not an infringement.
Moreover, even if the mark is protected in Finland, trademarks generally protect only goods in the same general market category. If "Jughead" is a trademark protected in Finland, it is presumably related to an entertainment publication, not to a restaurant. However, if reasonable consumers could plausibly believe that the restaurant is sponsored, endorsed, or approved by the publishers of the comics, that would also be an infringement, provided that the mark is protected in Finland.
Use of any image from or based on the comic figure (without permission) would be an infringement of copyright. Use of the name would not.
Use of the name might be an infringement of a trademark in Finland, IF the name is protected as a trademark in Finland. Significant research, beyond the scope of this site, would be needed to determine if such protection applies. A Finnish trademark lawyer might be able to help with that.