Different result per language

Use search_api_solr_devel and/or the analyzer (part of search_api_solr_admin) to see what is going on.
You could also post the output here.

Hi mkalkbrenner,
this is my output

English search

Index: default_global_index
Keys: 'PMD05663'
Parsed keys: array (
    '#conjunction' => 'AND',
    0 => 'PMD05663',
  )
Searched fields: brand, field_codici_alias, machine_model, machine_type, rendered_item, sku, title, zone
Searched languages: en
Conditions:
  status = 1
Sorting: search_api_relevance DESC
Options: array (
    'search_api_view' => 'object (Drupal\\views\\ViewExecutable)',
    'search_api_included_languages' => 
    array (
      0 => 'en',
    ),
  )

Solr request rawData

omitHeader=false&TZ=Europe%2FRome&echoParams=all&wt=json&json.nl=flat&q=%7B%21boost+b%3Dboost_document%7D++%28tm_X3b_en_brand%3A%28%2B%22PMD05663%22%29%5E5+tm_X3b_und_brand%3A%28%2B%22PMD05663%22%29%5E5+tocedgestringm_X3b_en_field_codici_alias%3A%28%2B%22PMD05663%22%29%5E21+tocedgestringm_X3b_und_field_codici_alias%3A%28%2B%22PMD05663%22%29%5E21+tm_X3b_en_machine_model%3A%28%2B%22PMD05663%22%29%5E5+tm_X3b_und_machine_model%3A%28%2B%22PMD05663%22%29%5E5+tm_X3b_en_machine_type%3A%28%2B%22PMD05663%22%29%5E5+tm_X3b_und_machine_type%3A%28%2B%22PMD05663%22%29%5E5+tm_X3b_en_rendered_item%3A%28%2B%22PMD05663%22%29%5E1+tm_X3b_und_rendered_item%3A%28%2B%22PMD05663%22%29%5E1+tocedgestringm_X3b_en_sku%3A%28%2B%22PMD05663%22%29%5E21+tocedgestringm_X3b_und_sku%3A%28%2B%22PMD05663%22%29%5E21+tm_X3b_en_title%3A%28%2B%22PMD05663%22%29%5E13+tm_X3b_und_title%3A%28%2B%22PMD05663%22%29%5E13+tm_X3b_en_zone%3A%28%2B%22PMD05663%22%29%5E5+tm_X3b_und_zone%3A%28%2B%22PMD05663%22%29%5E5%29&start=0&rows=20&fl=ss_search_api_id%2Css_search_api_language%2Cscore%2Chash&sort=score+desc&fq=bs_status%3A%22true%22&fq=%2Bindex_id%3Adefault_global_index&fq=ss_search_api_language%3A%28%22en%22+%22und%22%29&debugQuery=true&debug.explain.structured=true&facet.field=%7B%21key%3Ditm_field_attribute_3_ref+ex%3Dfacet%3Afield_attribute_3_ref%7Ditm_field_attribute_3_ref&facet.field=%7B%21key%3Ditm_field_attribute_2_ref+ex%3Dfacet%3Afield_attribute_2_ref%7Ditm_field_attribute_2_ref&facet.field=%7B%21key%3Ditm_field_brand_ref+ex%3Dfacet%3Afield_brand_ref%7Ditm_field_brand_ref&facet.field=%7B%21key%3Ditm_field_attribute_4_ref+ex%3Dfacet%3Afield_attribute_4_ref%7Ditm_field_attribute_4_ref&f.itm_field_attribute_3_ref.facet.limit=-1&f.itm_field_attribute_3_ref.facet.missing=false&f.itm_field_attribute_2_ref.facet.limit=-1&f.itm_field_attribute_2_ref.facet.missing=false&f.itm_field_brand_ref.facet.limit=-1&f.itm_field_brand_ref.facet.missing=false&f.itm_field_attribute_4_ref.facet.limit=-1&f.itm_field_attribute_4_ref.facet.missing=false&facet=true&facet.sort=count&facet.limit=10&facet.mincount=1&facet.missing=false

Italian search

Index: default_global_index
Keys: 'PMD05663'
Parsed keys: array (
    '#conjunction' => 'AND',
    0 => 'PMD05663',
  )
Searched fields: brand, field_codici_alias, machine_model, machine_type, rendered_item, sku, title, zone
Searched languages: it
Conditions:
  status = 1
Sorting: search_api_relevance DESC
Options: array (
    'search_api_view' => 'object (Drupal\\views\\ViewExecutable)',
    'search_api_included_languages' => 
    array (
      0 => 'it',
    ),
  )

Solr request rawData

omitHeader=false&TZ=Europe%2FRome&echoParams=all&wt=json&json.nl=flat&q=%7B%21boost+b%3Dboost_document%7D++%28tm_X3b_it_brand%3A%28%2B%22PMD05663%22%29%5E5+tm_X3b_und_brand%3A%28%2B%22PMD05663%22%29%5E5+tocedgestringm_X3b_it_field_codici_alias%3A%28%2B%22PMD05663%22%29%5E21+tocedgestringm_X3b_und_field_codici_alias%3A%28%2B%22PMD05663%22%29%5E21+tm_X3b_it_machine_model%3A%28%2B%22PMD05663%22%29%5E5+tm_X3b_und_machine_model%3A%28%2B%22PMD05663%22%29%5E5+tm_X3b_it_machine_type%3A%28%2B%22PMD05663%22%29%5E5+tm_X3b_und_machine_type%3A%28%2B%22PMD05663%22%29%5E5+tm_X3b_it_rendered_item%3A%28%2B%22PMD05663%22%29%5E1+tm_X3b_und_rendered_item%3A%28%2B%22PMD05663%22%29%5E1+tocedgestringm_X3b_it_sku%3A%28%2B%22PMD05663%22%29%5E21+tocedgestringm_X3b_und_sku%3A%28%2B%22PMD05663%22%29%5E21+tm_X3b_it_title%3A%28%2B%22PMD05663%22%29%5E13+tm_X3b_und_title%3A%28%2B%22PMD05663%22%29%5E13+tm_X3b_it_zone%3A%28%2B%22PMD05663%22%29%5E5+tm_X3b_und_zone%3A%28%2B%22PMD05663%22%29%5E5%29&start=0&rows=20&fl=ss_search_api_id%2Css_search_api_language%2Cscore%2Chash&sort=score+desc&fq=bs_status%3A%22true%22&fq=%2Bindex_id%3Adefault_global_index&fq=ss_search_api_language%3A%28%22it%22+%22und%22%29&debugQuery=true&debug.explain.structured=true&facet.field=%7B%21key%3Ditm_field_attribute_3_ref+ex%3Dfacet%3Afield_attribute_3_ref%7Ditm_field_attribute_3_ref&facet.field=%7B%21key%3Ditm_field_attribute_2_ref+ex%3Dfacet%3Afield_attribute_2_ref%7Ditm_field_attribute_2_ref&facet.field=%7B%21key%3Ditm_field_brand_ref+ex%3Dfacet%3Afield_brand_ref%7Ditm_field_brand_ref&facet.field=%7B%21key%3Ditm_field_attribute_4_ref+ex%3Dfacet%3Afield_attribute_4_ref%7Ditm_field_attribute_4_ref&f.itm_field_attribute_3_ref.facet.limit=-1&f.itm_field_attribute_3_ref.facet.missing=false&f.itm_field_attribute_2_ref.facet.limit=-1&f.itm_field_attribute_2_ref.facet.missing=false&f.itm_field_brand_ref.facet.limit=-1&f.itm_field_brand_ref.facet.missing=false&f.itm_field_attribute_4_ref.facet.limit=-1&f.itm_field_attribute_4_ref.facet.missing=false&facet=true&facet.sort=count&facet.limit=10&facet.mincount=1&facet.missing=false

There are no difference.

With search_api_solr_devel, you also get detailed output of Solr answers. These will give us more information.
You could also add the score debugging to your views output.

no further feedback