That's true, at the end of a day only Germanic and Slavic tribes were left in the area. We can only suspect and make cultural connection through archaeology.
This is extent of Celtic influence of Hallstatt culture.
There might be Celtic substratum left in pronunciation of some sounds. ʒ́ ć ź ś I can hear them in Portuguese, Irish and of course Slavic.
I think there should be some substratum in Slavic languages because they had expended over some existing populations. I think Polish language is harsher than other Slavic languages possibly due to Germanic substratum. Slavs from Balkans might have some Roman language substratum, or whatever form of Latin was spoken there. Czechs and Slovaks might have some Celtic pronunciation and rhythm left. Ancient population substratum can explain verity of existing dialects in languages.
čćžšđ are often result of
http://en.wikipedia.org/wiki/Palatalization and
http://en.wikipedia.org/wiki/Iotation in slavic languages, its part of our language law.
They are also purebred sounds, with their own letters, and are part of words, no matter the above.
That means they are not skewed or modified sounds in words, like kentum languages work(and i mean modern germanic and latin languages), because our language doesnt have pronunciation laws, you read every sound as it is written, no exception
That means you cant have situation like for instance, Jack and George, start with different letter that sounds the same, to us it woud al be Đđ; Đek and Đorđ.
We also dont write letters we dont pronounce.
That robustness of our language is actually how original latin was as well, also it makes it easy to recognize foreign words in our diction, because our root of our words are strong and meaningful.
So for instance, stvaranje(creation), has a root word stvar(thing), which is also made from tvar(substance), as well as others like stvor(creature), tvornica(factory)...etc
So from English words above, you can only be sure that creation, and creature are from same language source, and others you need to research.
So you have no support for such claim, i mean just look at your map
Also, if celts were actually people of same origin, not some Roman designation, then you should look western Europeans who got conquered by Romans and Germans respectably, and try to find traces of Celtic influence in modified Latin or old Germanic, like French, Spanish, English... etc, that seems most plausible from historical and genetic perspective.
As for Galitia, you have allready been told its a Latinized version of word that has nothing to do with Gauls