#1. Glue vas needed on some parts of the shoes. The glue was obtained by heating of birch bark as in a birch-bark bucket.
BERTZ is the Basque word for BUCKET. A feature of the Basque Language(s) is that a Basque word may not begin with more than a single consonant. In Slavic languages you may have a long line of consonants without any vowels in between.
The Czech and Slovak word for a BUCKET is DŽBER. The Slovenian word for BUCKET is ČEBER.
#2. The RADIANT HEAT of a FLAME is GAR in Basque. FLAME'S RADIANT HEAT is ŽÁR in Slavic languages.
#3. GORI is the Basque word for BURNING. The Slavic words for BURNING are GORI, GORETI, HOŘÍ, HOŘETI.
#4. The Basque word for glue is kola. The Czech word for glue is klih. The Slovenian word for glue is klej. To the extent that KLIH, KLEJ and KOLA somewhat resemble the English word GLUE -- I put a low premium on this. All the words contain a GL or a KL.
#5. Otzi's shoes were sewn and bound together with a TWINE. TWINE is MATAZA in Basque. TWINE is MOTOUZ in Czech. TWINE is MOTVOZ in Slovenian.
#6. Otzi's shoes had been PATCHED. The Basque word for PATCH is TXAPLATA (pronounced Chop lot ah).
The Czech word for PATCH is ZÁPLATA.
#7. GERUZA is the Basque word for LEATHER or SKIN or BARK. The Common Slavic word is KOŽA or KORA- KŮRA.
#8. BELAR is the Basque word for GRASS. BILINA is the Common Slavic word for GRASS, such as was used for insulation for Otzi's shoes.
#9. The knives and blades for cutting leather had to be SHARP. SHARP is OSTRO in Slavic languages, and ZOROTZ is SHARP in Basgue.
Of all the Slavic nations and their languages the Slovenes and the Czechs are the Most Western, and therefore geographically closest to the Basques of northern Spain and south-western France. I speculate that in the Stone Age there was a trade language as Swahili is now in sub-Saharan Africa.