Are Germanic languages closer to Italo-Celtic languages or Balto-Slavic languages?

Question

I ask because in some recent classifications, Italo-Celtic languages (like French, Spanish, Italian, Irish, and Breton), Balto-Slavic languages (like Lithuanian, Russian, Polish, and Serbo-Croat), and Germanic languages form the North-West Indo-European languages.

Answer 1

The best answer is: There is no consensus about this. In the big tree of Indogermanic languages there are only two intermediate groupings that are generally accepted: Indo-Iranian and Balto-Slavic. The support of Italo-Celtic became weaker with the discovery of Tocharian: Some of the common features of Italo-Celtic also occur there and are now considered archaisms instead of common Italo-Celtic innovations.

There is also a big split between Anatolian (containing Hittite and Luwian and some more languages that died out already in antique times) and the whole rest of the languages that is generally accepted.

Anything else varies from author to author, or from study to study and depends heavily on the choice and preparation of the input data. There are always isoglosses that cross proposed boundaries or nodes: It is more like a dialect continuum that fell into pieces than like one language undergoing a series of binary splits.

Answer 2

These issues of subgroups are often polluted by underlying political motivations, for example French linguists would favor an Italo-Celtic branch, German linguists would not like a Germanico-Balto-Slavic branch, English (speaking) linguists would favor a Celto-Germanic branch, etc.
In my opinion, Germanic is objectively closer to Balto-Slavic: for example the distinction between a and o is weak in both groups, even though the confusion is not patterned in the same ways. Another issue is declensions and here again Germanic shares a number of points with Balto-Slavic that are absent from either Italic or Celtic (like m instead of bh for some oblique cases). Italic and Celtic also share a verbal morpheme -bh- for unrealized actions (future or optative). Again this morpheme is not in Germanic or Balto-Slavic.
So on several points (phonetics, nominal or verbal morphology) Germanic sides with Balto-Slavic, not with either Italic or Celtic.
On the whole, Germanic looks like a kind of Balto-Slavic language that has undergone extremely strong modifications, under the influence of non-Indo-European substrates in Northern Europe: the first mutation of consonants, the loss of stress mobility, etc.
What Germanic shares with Italic or Celtic is mostly a number of substratic words, that are usually not present in Balto-Slavic nor in most other Indo-European languages.