england-and-wales scotland
Heraldic Law and Jurisdiction of armorial assumption
<p>In countries where armigers (persons or corporations bearing a coat of arms) must prove their entitlement to such (eg. most legal traditions within the UK, Canada, and other Commonwealth realms) what laws govern armigers bearing foreign arms (eg. a German, French, or American armiger in the UK)?</p>

<p>I am not sure 'laws' is the appropriate term: though the <a href="https://en.wikipedia.org/wiki/High_Court_of_Chivalry" rel="nofollow noreferrer">Court of Chivalry</a> still exists it has sat only once in the last 300 years (to determine whether a theatre had the right to display the municipal arms). The decision as to who is entitled to display a coat of arms is made by the heraldic authority in each country (the <a href="http://www.college-of-arms.gov.uk" rel="nofollow noreferrer">College of Arms</a> for England and Wales and much of the Commonwealth; Lyon King of Arms for Scotland).  These authorities work closely with each other (unsurprisingly when you consider that one of their original functions was to broker peace treaties), so it is likely that if arms granted in one country will be recognised by the heralds in another; but the decision is made individually.</p>

<p>I found one interesting quote, though my Google-fu does not allow me to identify the source:</p>

<blockquote>
  <p>M . J. Sayer writes some interesting things about registrations of
  foreign arms with the College of Arms in his "English Nobility: The
  Gentry, the Heralds and the Continental Context" (Norfolk Heraldry
  Society, 1979).  On pages 17-18 Sayer writes "where foreign arms are
  recognized in England, they of course rank in England as ensigns of
  nobility, even if the family was not noble abroad, an anomaly
  reflecting the greater success of the English crown's control over
  arms."  A footnote continues: "Sir Anthony Wagner kindly informs me
  that 'foreign arms for which the authority of a document from a
  Sovereign or heraldic authority can be produced have a head start
  towards recognition here over Burgher arms.  Nevertheless, though
  acceptance of the document may be the first stage, it has often been
  thought necessary that there should be a second stage of confirmation
  of the right to use the arms in this country.  One sees that there
  could in theory, and sometimes in practice, be a clash with the design
  of existing English arms.  At the present day we keep a record
  entitled Foreign Arms where such documents are entered when accepted
  as valid, but over and above this there needs to be acceptance by
  Patent or otherwise for use in England.' (30 March 1978)...Grants by
  foreign authorities to persons not within their jurisdictions, i.e. to
  nationals of a third country, are not accepted for registration."</p>
  
  <p>The College of Arms has at times contacted foreign governments to
  ascertain the status of their native heraldic authorities.</p>
</blockquote>

<p>Finally, I am surprised by your reference to American armigers; it is my understanding that the Article I of the US Constitution forbids the states to create titles of nobility.</p>
