Decrease width of language select, pull right

- @ogerly hope you are cool with this, I pulled it right so that the
validations you added still all line up on the right side

- @alina-beck I think you know a more sophisticated way to do this, it
was just easy for me to add an empty <ds-flex-item> with a 50% width on
medium and larger devices. This can also be populated with country of
contribution or visibility, when one of those is implemented.. what do
you think?

webapp/components/ContributionForm/ContributionForm.vue

@@ -61,6 +61,7 @@
           <ds-chip v-else size="base">{{ form.categoryIds.length }} / 3</ds-chip>
         </ds-text>
         <ds-flex class="contribution-form-footer">
+          <ds-flex-item :width="{ lg: '50%', md: '50%', sm: '100%' }" />
           <ds-flex-item>
             <ds-space margin-bottom="small" />
             <ds-select